Hello guys.
I am mining with 2x unlocked XFX 6950's. Lately I've been trying to find an overclock that's stable with 63% fan speed (night-mode), but all of them are pretty much unusable, and the way it crashes the system is very weird - on a "bad" overclock firstly the cards heat up to working temperature (between 65-80 depending on mV and mhz) and then stay there for a while (from 1 to 60 minutes); then, suddenly temperature drops on a single card, and the hashrate halves. If I move my mouse, the computer hangs. If not - the second GPU continues to mine for a little longer (2-3min) before a total freeze.
What surprised me the most, however, was results I got from last night: I let both of them run @ 1100mV(stock voltage) and with 900mhz core/625mhz mem and went to bed after 20-30mins of it being stable. It ran smoothly for an hour or so (according to BTCGuild logs) and crashed an hour later. I surely thought running it on stock voltage and such a low overclock (stock max is 840mhz core) will help the stability. I am sure it crashed the same way (single gpu halting for no reason, and then, even with 99% usage dropping temperature until a full system crash).
What could be the reason for such weird behaviour? What kind of overclocks do you have on your 6950s?