Unexplainable CPU utilization

[Particle]

I've never seen anything quite like this. On my system I was experiencing CPU usage that I couldn't explain. CPU idle time would be reported as 99% in task manager, but CPU utilization would be something like 5%, 10%, even 15% constantly. The numbers don't add up. When this is happening, audio playback is often jerky. The loading doesn't appear to be anything "real" as all CPUs are hit perfectly equally. I don't know what could be causing this right off, as there aren't any interrupts or DPCs even taking up CPU time.

I reinstalled and this behavior quit. It's back now though. Interestingly enough, the amount of CPU time is like a smooth rolling sine wave. It'll go between 0% and 20% and back down to 0% over the course of about 10-20 minutes.

I've attached some images demonstrating the discrepancy. Anyone have any idea where to look?

[Particle]

One more image that shows a more advanced tool that details DPC and IRQ.

[zanzabar]

do u have cool and quiet or EISt/speedstep, that will glitch the task manager usage history by changing the number of cycles so the idle percentage will shoot up, but the usage based on cashe interaction will stay low.

and if u had an old install set to high performance it will stop the cpu from down clocking with hardware power saving mode enabled, but the default windows settings encourage down clocking

[SocketMan]

Boot up in safe mode and check the task manager (for cpu
utilization)
You can also add the columns with i/o read/write bytes/per second to the task manager.