Problems with Nvidia GeForce 8800 GS

[Glenarvon]

I'm running an iMac under OS 10.5.8, Intel Core 2 Duo, 3.06 GHz, 2GB Ram. The troublesome graphics card is a NVidia GeForce 8800 GS.

Recently, my graphics card died and I got it replaced. However, since then the computer is behaving not as it should. I got weird graphical glitches, bits of the screen were "displaced", dock icons had a bright green background... also the system would randomly freeze. However, all of this would only happen about once a week.

I repaired both permissions and volumes several times (booting from disc to do that) and it seems to have helped the worst of those problems. Still, graphics intensive games (e. g. The Sims 3, Assassin's Creed, Prince of Persia) still show extremely odd glitches. These happen randomly, but a lot more frequently than once a week.

The system also seems to run rather hot when glitches happen, around 70° C, though I'm not sure whether that's a symptom.

It should be pointed out that all games were running perfectly smoothly before the graphics card was replaced and the computer hardly ever crashed.

I suspect that either the replacement card was already broken from the start or was incorrectly installed. However, in both cases, shouldn't the problems be more persistent? Could it be a driver problem?

I'd rather not bring it back to the repair shop since I primarily work with this machine rather than play, but I want it to be fully functional and as reliable as it was before.

Stuff I already did:
- repaired permissions and volumes (several times)
- reset NVRAM and PRAM (twice)
- run Apple Hardware Test (came out clean)

I hope someone can give me an idea of what the hell is going on. Thanks!