Tried this?
Q: The CPU usage is high.
A: The ATI drivers after 11.6 have a bug that makes them consume 100% of one
CPU core unnecessarily so downgrade to 11.6. Binding cgminer to one CPU core on
windows can minimise it to 100% (instead of more than one core). Driver version
11.11 on linux and 11.12 on windows appear to have fixed this issue. Note that
later drivers may have an apparent return of high CPU usage. Try
'export GPU_USE_SYNC_OBJECTS=1' on Linux before starting cgminer. You can also
set this variable in windows via a batch file or on the command line before
starting cgminer with 'setx GPU_USE_SYNC_OBJECTS 1'
I forgot to mention - yes I did try that before I made the post. It had no effect. The rig was running on Catalyst 12.8 before I reformatted it, although I don't remember the SDK version I was using. I'll start playing with the SDK version and see if that fixes it.