Games randomly low framerate bursts. I seriously need help.

This is really getting on my nerves. My games will randomly lag for anywhere from 2 minutes - 4 minutes down to 5 frames per second, and then they will be back to full speed. I have no idea what is doing this.

It happens for:

-Call of Duty 4 multiplayer

-Sid Meier's Pirates (Not online)

-Frontlines: Fuel of War beta

-Company of Heros (Not online)

However, it doesn't happen with Call of Duty 4 singleplayer (which I thought was very strange).

Could it be?:

-Ram issue

-Video Card issue

-Problem from CPU

-Problem with CPU because I did overclocking wrong

I've already defragged my computer, and I've done multiple virus and spyware scans. When I use CPU-Z my CPU clock jumps from 1.8ghz-2.4ghz within seconds of each reading. Can anyone help?