EXCEL 2010 taking lots of CPU time in spurts

Lately I've been seeing performance problems on multiple workbooks. What happens is I'll be working away and then EXCEL just stops responding. I can't select cells, scroll screens, ..., nothing, really. This condition will last for maybe 30 seconds and then stops and I can work for a while. During that down time I occasionally see that spinning "Wait" indication pop up for a second or two. I opened the task manager and see that Excel is taking around 22% of the CPU while it's ignoring me.

I can't nail down any cycle time, though. These spurts will happen every 5 - 10 minutes. And I don't see any correlation to what I'm doing. Also, as I mentioned, it happens on multiple workbooks, so it doesn't seem to be sensitive to content.

Any idea why Excel would just "go busy" on me once in a while? This has been going on for the past three or four days. Prior to that I never saw this.