I've been mining for quite some time, but lately one of my GPUs (an ATI 5850 OC) started locking up after an average of 12 hours. I'm wondering what I could do to avoid these lockups and what the causes are.

The problem started when I switched to the phoenix miner (from Diablo), I think it might be due to a higher usage of the cores (since Diablo in my case was slowed down by CPU, no idea why), so while I now get the advertised hashing speed this single core locks up, making the whole thing not very productive after all.

Any idea?