I use Radeon 5850, app sdk 2.5, catalyst 11.7 drivers. The overall performance increased significantly since 2.4/11.3 -- in our video-processing application it grew from 1.45 to 4.17 fps.
However, after some time the performance drops down more than twice, sometimes even back to 1.4 fps. I also noticed that when this happens, GPU clock stays at 400 MHz and does not increase under load (unlike memory clock that raises from 900 to 1000). So far the only way to recover I found is to reboot.
Is it a known issue?
After some more experiments I found out that in some cases I get poor performance even in situations when GPU clocks are properly adjusted.