I have the same problem, although it is uncommon, and only happens when I run 5 copies of the game.
4 or less it never happens, so I am assuming its RAM related.

my system is completely different, Q9650, 9800GTX, 4gb ram, oc'd to 1589FSB.
I gonna pinch 4gb out of my other machine and test it soon, so will report back when I do.
edit: bit more info.

I run 1 full screen, 1920x1200, max fps possible, usually around 40fps
the other 4 are on a screen the same, locked to 30 fps
when this happens, only my main screen drops, and drops to about 10fps, if I switxh them around, still only my main one drops.