Macbook Pro w/ 8GB frequently hangs for 15 seconds

My lovely wife's 2009 Macbook Pro with Lion 10.7.4 has a bizarre hanging condition. It will frequently hang with the beach ball of doom for about 10-20 second at a time.

I have installed 8 GB of RAM, most of which is free on Activity Monitor.

Activity monitor shows zero CPU usage and zero network usage and zero disk usage when it hangs. It displays the hanging app as "not repsonding" in red.

The hang usually occurs after changing to a new window, but will occur in any application (iPhoto, iMovie, Word, Excel...) or even Finder.

I recently had iMovie do a long thumbnail creation task, and the CPU monitor showed 10 seconds of much activity then 15 seconds of no activity in in a repating pattern.

I have a mid 2011 MBP with the same configuration and it behaves just fine.

I'm at my wit's end, since there is no way to know what the CPU is thinking about when it hangs.

Ideas?