Any ideas for me to try?  Or what could be the problem?

I have 3 6970's, which previously I had 2 in a ubuntu machine and 1 in an xp box.  I was averaging 330 m/hashes per gpu.  Today I put all 3 in my ubuntu box and now I average 280 - 300 m/hashes.  Why the drop in hashing power and is there anything I can do to regain the original hashing power?