Vostro 230 - Won't Boot Up Properly

Vostro 230 slim desktop, Win 7, dual drive (SSD and HDD)

about a month ago, I started having a strange bootup issue... never had it before

ISSUE:

if I leave it overnight, then try to start up, it fails with these symptoms:

- both the CPU fan and PSU fans immediately start, and continue running properly

- hard drive activity light shows NO ACTION at all and stays off (should light up blue and flicker)

- the main power button turns on solid blue and stays that way (no error codes, no blinking, no amber)

- the keyboard lights do NOT turn on (eg. Num Lock)... nothing I do with the keyboard (ESC, ctrl-alt-del, etc.) has any result... keyboard is effectively dead

- monitor gets NO SIGNAL, the power button light stays in standby (solid amber, vs. solid blue)... so obviously screen is BLACK

- pushing main power button does nothing... it just stays solid blue

- so I get stuck, and the only "way out" is to pull the black power plug from back of the PC

- surprisingly, if I then wait a bit (eg. 10-30 seconds) then plug the cord back in, the PC starts up all by itself, even though I do not push the power button!... it initially goes to the plain text "Windows Error Recovery" screen, and I choose "Start Windows normally"... then machine continues to boot up normally, and runs fine thereafter

- interestingly, if I try putting the machine to sleep during my workday, then restarting say 1 hour later, it re-awakens and runs fine, without any bootup issues at all

- the power supply has a green LED status light, which is ALWAYS solid green, even when machine is powered/shut down (there is no BIST test button on this LiteOn power supply model, only the single green LED)

- note: aside from this bootup issue, the machine runs fine for hours on end

any ideas? thank you!