This is what I would suspect, but who knows? I am convinced that there is a bug in DiabloMiner. What is your setup? card / driver / sdk / os?
it's a single 6870 from sapphire, using sdk 2.4, ubuntu 11.04 (I like unity, even though it might put some load on my gpu) and apt says my driver is "fglrx 2:8.840-0ubuntu4".
No manual over/underclocking.