Unknown CPU usage

P4 3E w/HT enabled
ASUS P4P800S mobo
2x512 Kingston RAM
ATI AIW X800XL AGP w/Zalman cooler @ 510/510
XP Home up to date

Task Mgr. shows my CPU cycles at 50% with the left window showing full throttle and the right one shows the normal spiking patterns from 0 to full depending on computer use. This usage shows up randomly during my normal computer use and I haven't been able to tag it to any program. I don't have any unusual or unknown processes running and, in fact, the System Idle Process shows 99% even while the performance graph shows the full usage pattern on the left. There is a noticeable drop in performance when this happens too.

I have turned off all unneeded processes until I was almost in safe mode and it still shows the 50% cycle. I haven't been using any particular program when this starts. It has occurred under at least 7 different programs. The only way I have been able to clear it is to restart. When I do, task mgr. shows only the normal, low, cycle patterns I'm used to until it suddenly and randomly shows up again.

Can anyone help me before I tear out what little hair I have left? confused