Yesterday, my Sapphire HD 6850's fan slowed down unexpectedly. When I checked, the fan's speed was between 300-500 RPM, which is extremely slow. So, I opened my case and removed the card from the motherboard and plugged it back in properly in the slot.
I turned on my computer and the card was fine and the fan's speed was normal again(~1300 RPM). But now, the card constantly freezes for a couple of seconds and then it runs fine for a few minutes. It just seems to lag whenever I'm browsing, watching videos or gaming.
What's wrong with it?
Try a stabilty test with OCCT. Dowload it at ocbase.com (sorry I can't give you a direct link). Turn error check on and put the time @1hr, 5min idle at start and 1min idle at the end. See how many errors it makes. If it works correcly it should just be 0.
It is also possible that the GPU suffered damage from the fan drop, which usually happens when the GPU get too hot and the PSU doesn't supply enough power for the fan. This means you should get a new power supply and GPU.