Have you updated your motherboard bios recently, or do they have a newer version available than what you have right now? I went through a fairly similar situation with my 5770 recently. Finally I updated the motherboard drivers to the latest ones again, and everything went back to normal and ran perfectly.
I tested 4 different motherboard bios drivers just to be 100000% sure that was the issue and not anything else. No matter what I did, unless I had the two newest ones (one is just a beta) the card would not run properly period.
Besides the 4 motherboard bios versions I also tried 3 different video driver packages, and used all of those in combination with the different motherboard bios versions (yes it took AGES, but I had to be 100% certain). In the end, if I wanted the video card to function properly I had to go with the newest motherboard bios version (or the even newer beta)....period.
The bad symptoms I was getting are as follows (not all at once, and some drivers would have some and not others)...
1. Watching ANY form of streaming flash video such as justin.tv would have lag to it, as soon as you go full screen the driver would fail within 5 seconds and cause a slight hang up before recovering. Once recovered the entire pc performance was horrible even at 3.8Ghz or higher...and I mean HORRIBLE like having lag+stutters just to get the taskbar up.
2. The display driver would fail while loading windows right after the welcome screen.
3. The card would show 99% usage in CCC (100% in GPU-Z) and would NOT clock down from default 850Mhz/1200Mhz clock to the powerplay clocks of 157Mhz/300Mhz. If you tried to do it manually it would register for roughly 2 seconds before reverting to 100% usage and high clocks. Trying to do anything graphical at that point would result in....well you get the idea.
4. If the system was running 'normal' for everything else and all was 'fixed' you could load up a game (for instance Crysis Warhead) and it would play fairly normally except...it would hiccup/stutter every few seconds making play of course very difficult. If you disabled Vsync it would then stop doing it every few seconds and make things appear to be running at say 20-25fps when the counter actually says 70-85fps. Hard to describe, but just imagine a game with a perfect frame rate...yet to your eyes it's stuttering every second making it feel more like 20-25fps.
I'm sure there were more things, but that will give you a good idea of what I mean. I don't know if you've checked, but it's worth a shot to check it out and see what happens for you. All is well now for me as long as I stay on one of these two newer motherboard drivers. (new as in November and December '09 versions, any version older than the card results in bad performance)
Kei
