I have similar issues with a single machine running a 5970. It works under linuxcoin but fails with ubuntu naughty. I also tried the maverick kernel, the debian testing kernel, various stream sdk versions (2.1, 2.4), different miners, various driver versions.

Me and the previous poster both use ubuntu naughty and because i tried different kernels (and also unloaded various modules etc.), it must be some part of the userland that is problematic.

The exact symptoms are kworker and python at about 100-200% on my dualcore. The events that are queued into the kworker queues seem to be quite normal, though.