Whats interesting from reading through this thread is that the problem exists on so many different hardware combos and OS's, and with different cards from the 5000 family, overclocked or not.
I mean I'm using a 5770 at stock speeds on AMD's own Dragon platform. (All of my main components are listed as Dragon certified). And it's just plain unusable.
I've been using ATi since the days of the Rage II, but if this is how their quality control works under AMD's ownership, then I'll be switching to nVidia.