Spanning tree protocol root changes problem

Hello!

I can not understand strange problem which is happening on our Powerconnect 6224.

We have small network. Powerconnect is our main switch, which is connected to our ISP. Every day in log file I see Spanning Tree Protocol messages like this:

      14       5304446     Notice     APR 24 00:03:34     TRAPMGR     %% New Spanning Tree Root: 0, Unit: 1       
      15       5304447     Notice     APR 24 00:03:34     TRAPMGR     %% Unit 1 elected as the new STP root      
      16       5304448     Notice     APR 24 00:03:34     TRAPMGR     %% Instance 0 has elected a new STP root: 8000:aaaa:bbbb:4ef3      
      17       5304449     Notice     APR 24 00:03:34     TRAPMGR     %% Unit 1 Port 1 is transitioned from the Forwarding state to the Blocking state in instance 0      
      18       5304450     Notice     APR 24 00:03:34     TRAPMGR     %% Unit 1 Port 1 is transitioned from the Learning state to the Forwarding state in instance 0      
      19       5304451     Notice     APR 24 00:03:35     TRAPMGR     %% Link Down: VLAN- 2      
      20       5304452     Notice     APR 24 00:03:35     TRAPMGR     %% Link on VLAN- 2 is failed      
      21       5304453     Notice     APR 24 00:03:35     TRAPMGR     %% Link Up: VLAN- 2      
      22       5304456     Notice     APR 24 00:03:35     TRAPMGR     %% Instance 0 has elected a new STP root: 8000:yyyy:cccc:3300      
      23       5304457     Notice     APR 24 00:03:35     TRAPMGR     %% Unit 1 Port 11 is transitioned from the Forwarding state to the Blocking state in instance 0      
      24       5304458     Notice     APR 24 00:03:35     TRAPMGR     %% Unit 1 Port 18 is transitioned from the Forwarding state to the Blocking state in instance 0      
      25       5304459     Notice     APR 24 00:03:36     TRAPMGR     %% Unit 1 Port 18 is transitioned from the Learning state to the Forwarding state in instance 0      
      26       5304485     Notice     APR 24 00:04:05     TRAPMGR     %% Unit 1 Port 11 is transitioned from the Learning state to the Forwarding state in instance 0

aaaa:bbbb:4ef3 is ours Dell MAC
yyyy:cccc:3300 is ISP main switch MAC (where our Dell on port 1 is connected).
The only member of VLAN2 is Port 1.
Switch configuration see below.

Why the STP root changes from one switch to another and then back again?

Configuration:

!Current Configuration:
!System Description "Dell 24 Port Gigabit Ethernet, 2.2.0.3, VxWorks5.5.1"
!System Software Version 2.2.0.3
!
configure
vlan database
vlan  2-3,24
ip igmp snooping 3
exit
hostname "SciDell"
sntp unicast client enable
sntp client poll timer 1024
sntp server 83.64.124.251 priority 2
sntp server 147.231.19.43
clock timezone 2 minutes 0 zone "EET"
stack
member 1 1
exit
ip address 192.168.2.1 255.255.255.0
ip address vlan 24
logging buffered size 300
ip routing
ip route 0.0.0.0 0.0.0.0 X.Y.Z.161
bootpdhcprelay enable
bootpdhcprelay maxhopcount 3
bootpdhcprelay serverip X.Y.Z.167
interface vlan 1
bridge multicast forbidden forward-unregistered
exit
interface vlan 2
bridge multicast forbidden forward-unregistered
bridge multicast forbidden address 0100.5E01.000C add ethernet 1/g1
bridge multicast forbidden address 0100.5E01.000D add ethernet 1/g1
bridge multicast forbidden address 0100.5E01.000E add ethernet 1/g1
bridge multicast forbidden address 0100.5E40.000A add ethernet 1/g1
bridge multicast forbidden address 0100.5E40.000B add ethernet 1/g1
name "Uplink"
routing
ip address  X.Y.Z.171  255.255.255.224
no ip proxy-arp
exit
interface vlan 3
bridge multicast forbidden forward-unregistered
bridge multicast address 0100.5E7F.FFFA
name "CPE"
routing
ip address  X.Y.Z.1  255.255.254.0
no ip proxy-arp
ip igmp
exit
interface vlan 24
name "Management"
exit
ip igmp
username "aaaa" password f6db467628f299049005d10cd6335569 level 15 encrypted
bridge multicast filtering
ip igmp snooping
!
interface ethernet 1/g1
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 2
exit
!
interface ethernet 1/g2
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g3
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g4
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g5
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g6
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g7
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g8
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g9
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g10
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g11
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g12
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g13
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g14
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g15
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g16
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g17
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g18
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g19
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g20
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g21
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g22
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g23
spanning-tree cost 20000
spanning-tree portfast
switchport access vlan 3
exit
!
interface ethernet 1/g24
spanning-tree portfast
switchport access vlan 24
exit
!
interface ethernet 1/xg1
switchport access vlan 3
exit
!
interface ethernet 1/xg2
switchport access vlan 3
exit
!
interface ethernet 1/xg3
switchport access vlan 3
exit
!
interface ethernet 1/xg4
switchport access vlan 3
exit
!
interface port-channel 1
switchport access vlan 2
exit
!
interface port-channel 2
switchport access vlan 3
exit
!
interface port-channel 3
switchport access vlan 3
exit
!
interface port-channel 4
switchport access vlan 3
exit
!
interface port-channel 5
switchport access vlan 3
exit
!
interface port-channel 6
switchport access vlan 3
exit
!
interface port-channel 7
switchport access vlan 3
exit
!
interface port-channel 8
switchport access vlan 3
exit
!
interface port-channel 9
switchport access vlan 3
exit
!
interface port-channel 10
switchport access vlan 3
exit
!
interface port-channel 11
switchport access vlan 3
exit
!
interface port-channel 12
switchport access vlan 3
exit
!
interface port-channel 13
switchport access vlan 3
exit
!
interface port-channel 14
switchport access vlan 3
exit
!
interface port-channel 15
switchport access vlan 3
exit
!
interface port-channel 16
switchport access vlan 3
exit
!
interface port-channel 17
switchport access vlan 3
exit
!
interface port-channel 18
switchport access vlan 3
exit
!
interface port-channel 19
switchport access vlan 3
exit
!
interface port-channel 20
switchport access vlan 3
exit
!
interface port-channel 21
switchport access vlan 3
exit
!
interface port-channel 22
switchport access vlan 3
exit
!
interface port-channel 23
switchport access vlan 3
exit
!
interface port-channel 24
switchport access vlan 24
exit
snmp-server group readr v1 read Default
snmp-server group readr v2 read Default
snmp-server community-group readr readr
exit