update: I did end up frying the card. The reason is monitoring voltage with 2 apps, like GPU-z, Everest and/or Afterburner can cause a bug in the 5xx0 cards to spike voltage to 1.65v!. Read below for details.
I have a 5850 with a huge Accelero twin turbo cooler at 100% and using stock cooler plate for ram and vrms.
Its been mining under ubuntu for 2 weeks with no problems, GPU below 50C, VRMS around 80C.
I booted into windows for some stuff, and kept mining. When I checked my temps, I got pretty much the same temps as in ubuntu. A while later I checked again, and suddenly I saw the left side of the chart:
OUCH
VRMS above 120C GPU throtteling at 100C!
Immediately stopped the miner, thats the first decline you see.
First I thought the fans must have died or something, but strangely, the temps dropped back to normal idle temps. So call me crazy but some time later I launched the miner again and kept watching. For 20 minutes or so, I got perfectly normal loaded temps again. GPU below 50C, VRMS around 80C. Then BANG, VRM and GPU temps shoot up again almost instantly.
WTF?
Card still seems to work fine. I dont get it.