When i am not using my 5770 in 3D applications it usually idles at GPU 157 and Memory 500. The card just goes 400 / 900 for no reason at all sometimes and is stuck at that frequency even when its idling or full load.
PS. Played Crysis for 25min at (awful FPS at that frequency) and the clocks stayed 400/900. Restart fixed it temporarily.
I know this thread has been inactive for a while, but I wanted to share my experiences. I have a powercolor pcs+ 5850 with a non-reference pcb. This causes the voltage to be locked at 1.15. If you flash a bios on this card that is over 1.15 volts, the core/mem clock will be locked to 400/900. By lowering the clock on the bios I created, I was able to eliminate the 400/900 issue. I would suggest finding out what the voltage for other cards with the same model as yours and undervolting it to match that number. It may resolve your issue.