Dude your right and the answer is NOT a Heat Issue Whoever said that doesnt know what they are talking about. It comes from the way that Ati Throttles their driver on the desktop so that they're considered to be a "Green" solution. (For those worried about
Power consumption.)the ATI 5770 is rated at 300MHz(18Watts) on the desktop. Try the solutions below. It's a fairly new thing but all the drivers have it now. here is the fix taken from an amd site on the fix..
Instead of fiddling with clock speeds, all you need to do is increase the TDR (timeout detection and recovery) setting in the registry. I set mine to 6 seconds a long time ago, and never had the problem again.
On a side note, my MSI Lightning 5870 with latest drivers, the idle core speed is 157 mhz. It used to be 300 though, not sure when that changed.