Artifacts while not playing

hi,
I have a radeon 4870 and catalyst 9.2, when I play CoD4 there are some artifacts (even though there card isn't hot it maxes at about 65). now the artifacts began appearing on the desktop too when I'm not playing anything, they look like little squares and they appear randomly on the screen, when I move it a bit (like scrolling in FF) they disappear and appear somewhere else.
when it happens I turn off the computer for a while and then turn it back on and usually the problem disappears for a while.
what's the problem? what ca I do? is the card a goner? (it's still under warranty but here in israel warranty is a complicated thing).
btw the card is OCed when playing (through the catalyst) but even if I try to back it own the artifacts remain, I'm using vista w/ aero and when I'm on the dewsktop the card is underclocked to 500/450 so I can turn down the fan (it's on 48 degrees).