Don't worry unless you see it happening alot running 1 game.
A cause can be task switching between them, its to do with running 2 apps that use your GFX and direct X. I run CS and stall some times.
Seen it on my set up as well:
X2-3800+(stock 2.0Ghz O/C @ 2.6Ghz on air), 2GB-DDR, NF4 mobo, 8800GT AMP(O/C'ed PCI-E), seagate 500GB & maxtor 250GB HDD, NEC 3520 & P109 DVD-RW, TX-650W corsair (52amps single 12V rail

) PSU