I know exactly what the problem is. My 5870 could clcok upto 1000Mhz core easy but sometimes it would crash in CS:S at 900Mhz or even at stock while watching some movie. The crashing was pretty random but downlocking the memory to stock helped and i could even run at around 950Mhz core - it was still crashing from time to time though.
I've tried just about everything, flashing to different BIOS, raising the stock 2D clocks to 400/400 or even to default 3D clocks, modifying BIOS etc etc but nothing fixed the issue completely. I'm actually happy i don't have that card anymore, i can atleast watch movies now.