Periodic frame rate drops in MNC and other games

I've been noticing this happening in Monday Night Combat and in a few other games like Killing Floor (both Unreal Engine games, so I dunno if that has something to do with it). I'll be playing the game just fine for a while when suddenly my frame rate drops completely. Things start to stutter and frames start skipping until it returns to normal a few seconds later. After that the game runs fine until about a minute later when it happens again. It does it every minute or two, making the game almost unplayable. This tends to happen much quicker in MNC than it does in KF.

These are my basic specs:

AMD Athlon 64 X2 Dual Core Processor 5000+ 2.61 GHz
2.00 GB of RAM
NVIDIA GeForce GT 240

They aren't great, but I can run other games just fine like Team Fortress 2 and Left 4 Dead without this happening. Does anyone have suggestions as to what I should do? I've switched off Vsync and reduce mouse lag and that hasn't helped, and I have also tried degragging my computer, my Steam games and lowering the game's visuals to no avail.

Some people have told me extra RAM would probably solve this (although, being on Windows XP, I'm limited to 4 GB anyway), but I wanted a second opinion here. Are there any other solutions? And how much RAM would be enough to stop this from happening in MNC?

Bump.

Does your framerate improve if you tab to desktop, wait 10-20s and tab back in? Sounds pretty much like a temperature problem (CPU and/or GPU clocking down due to temperature).

If it happens, reboot and check temperatures in your BIOS (tools not always read them correctly). If experienced, open your case and remove any dust especially inside/below fans and heat sinks.