I have an XFX 5870. I've had it for nearly a year now. Within the last month or so It has started having problems. Games are crashing now and locking up and sometimes recovering. Its not overheating it never even gets to 50c when I have the fan on about 40%. It is slightly overclocked but seriously its by like 50MHz the stock clocks are 850/1200 mine are 900/1250 and It's been like that awhile so that isn't the problem. Usually it locks up and I have to hard reset it. Or Ctrl +Alt+Delete. One time it said in the task bar that the display drivers have stopped responding and recovered. The screen tears on the desktop sometimes and usually when I watch full screen HD video. Games started crashing too. I have Windows 7 Pro x64 and I'm using Catalyst 10.11 so WTF should I do its very annoying.
There seems to have been a problem with the 5870's when it comes to the VRM's. My 5870 also had these issues, and it was due to the VRM's overheating. The Thermal tape over the VRM's was fried to a crisp.
To help control the VRM temps, you need to underclock the RAM. You will likely want to replace the thermal tape too. The temps of the VRM are not monitored, which is why you are unaware of this issue. Never overclock the memory on these cards. Unless you have aftermarket cooling on the VRM's.
I forgot about this but here I am again. I set the default clocks 850/1200. And reinstalled Catalyst 10.11. And magically the screen tearing stopped during movies, and the games dont crash.. as much. Dead Rising 2, Bioshock, Fallout New Vegas, all dont crash at all. But Assassins Creed 2 and Need For Speed Shift still seem to lock up randomly, crash, or go to that weird grey patterned screen. Which is most likely the games problem not the GPU.