Computer lags intermittently while playing games

[Carade]

Now whenever I play Left 4 Dead 2 my computer will switch from running smooth as Fonzie, to lagging, and back to running smoothly. I've tried decreasing the graphical eye candy, and even disable multicore rendering, the problem isn't confined to Left 4 Dead either, I've encountered it during single player games such as Modern Warfare 2, also.

System Specs:

Processor: 2.13GHz Intel Core 2 Duo P7450
Memory: 4GB, 800MHz DDR2
Hard drive: 320GB 7,200rpm
Chipset: Mobile Intel PM45 Express Chipset
Graphics: 512MB Nvidia GeForce 9800M GS

Any help would be appreciated.