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.
Hmmm... I have a different setup, 2x sapphire 5830s, 2.4 SDK, 11.5 driver, debian squeeze. My cards are overclocked to the default bios max (875 mhz). This may be a more serious bug if it's affecting two very different setups. The only common thread here is the SDK and the platform (kinda).