I build a mining rig with two HD6870 graphics cards, which ran with 260MHashs/s each until yesterday. Since then the second one only runs at about 500kHashs/s which is unacceptable. I tried diffrent things like disabling crossfire (which it was at first, but then magically activated itself... well it certainly didn't but I don't know how it did), changing ubuntu themes and other useless stuff.
I am running ubuntu 11.04, installed it with the tutorial of Raulo (http://forum.bitcoin.org/?topic=3359.msg47174#msg47174
). Installed Catalyst driver version is 11.4 although I tried 11.6 but it didn't work. Oh and I am using poclbm as miner with -v -w128 if this is anything worth as information...
Yesterday I also had problems with freezing system when switching off the monitor. The problem is gone, but might be related.
Anybody has a clue? Might the graphics card be broken (it is not two weeks old)?