Mouse stops working in Optiplex 390 MT

Hi all,

From couple of months (almost all a year) I am facing one strange problem in my  Optiplex 390 MT  windows 7 64 bit pro installed. My mouse or keyboard (either of them)  keeps disconnecting randomly . It does not get disconnected at a time ( Mouse and keyboard) .   oneday mouse stops working and i see no laser light at the bottom of mouse .Another day it may be a keyboard stops working . What i do is ,,just i remove its connection and plug in to other USB ports at rear  and it works fine for few days . I tired all these below options.

1)  Clean installation of OS 

2)  Removed CMOS cell and re inserted cell after 30 mins.

3) Shortened the open pins of all jumper available in mother board in power off condition. (shortened for 1 min).

4) downloaded  latest drivers for all devices 

5) Ran dell hardware diagnostic tools and found nothing. 

6) Ran antimalware  (if  some virus is causing trouble)

7) Flashed bios STARTING FROM VERSION A03 to A11  . One by one.

8) changed many keyboards and mouse  ( i could have bought new computer in the money i spent for keyboard and mouse)

I did all these above things from a year and no result .

What i noticed in DEVICE manger is " Mouse or keyboard " does not show in the list when it any one of them gets disconnected.  I am sure it is not hardware issue .Because  i connected a  USB memory to that ports and it just works fine.  WHen keyboard gets disconnected then then  nothing works on key board during  bootup  ( F2 , F12). Totally keyboard remains disconnected. 

So i am totally exhausted and posting here to seek any help