Phoenix kept doing this for me, randomly. Something nothing, sometimes it would eat 100% CPU. Stopping & starting over and over sometimes fixes it.
Then again, poclbm never does this. I've been using that instead of Phoenix.
Do you use one card or two? Once I started using two cards with poclbm (and with Catalyst 11.6B) I got the CPU error with it as well. Prior to that I was using 11.6 on one card without the error.