doubt its CPU. I run a 5830 on a P4 prescott 2.8 @ 10% CPU usage. XP SP3, SDK 2.1, 11.2 driver (note: you dont want this combo of driver/sdk, its just an example of what usage I get with an anemic CPU).
what miner and settings? driver and sdk version? OS?
any other software on the rig?
task manager (if win based) shows it the miner @ 100%, and nothing else using CPU?
guiminer, poclbm -v -w256 -f1 (could it be the -f1?)
Task manager shows poclbm at 98% and other software at 2% (firefox)
I'll check the driver and sdk and report back.