I had a similar problem when I had my GTX260 in my old PC. I didn't realise for a long time that the power supply I had did not have sufficient amp requirements to power the graphics card. It was okay for the first couple of months, but once the efficiency degraded it would not power everything properly. (Crashing, artifacts) I ended up getting a decent power supply instead.
FYI: The GTX260 requires a power supply capable of 36amp on the 12v line for the whole system (I realise that they sometimes overstate this figure, but it is still a good guide to go by)