Computer freeze - have shut down then restart...

In the last 2 month, my PC freeze with a screen glitch (lines with snow) while i'm playing my game (BF2)
- it sometimes freeze on the desctop with the screen ok or with Civ 4.

i cannot reset; only way is to shut it down then restart.
then it can work properly for awhile before it happens again.

When i start my PC, the bios sometimes give me this message:

'Overclocking failed! F1 to change setting or F2 to continue'

I tested my memory and it pass all the test.

Here is my rig:

Intel P4 3Ghz HT
Asus P4C800E-Deluxe
ATI X800XL 256Mb AGP
2x 512MB DDR400 dual channel
300 watts power supply

P.S.: i tried changing my graphic card; still having same problem. I also cleaned the eatsink of my CPU about 2 days ago (didn't freeze since, but it's a bit soon to say it is solved).

PLEASE HELP!!!