Slow computer; CPU or GPU?

First of all, I know a few things about computers but I'm not sure what to do. That's why I am asking you guys. My computer seems to have major lag issues when it comes to playing games or even videos. However, on occasion it will stop lagging for about five minutes or so, completely lagless, but then the lag starts up again for 10-15 minutes. These lag spikes occur in whatever I do, games or videos. The small momentary lapses where it doesn't lag has led me to believe that it is probably possible to make my laptop always run like that. It is running Windows 7 64-bit, has 4 GB ram, the CPU operates at 2.0 GHz, and the GPU is the Nvidia 8200m g. I've tried over-clocking my GPU after finding out that it is a fairly low-end graphics hardware, but it yielded no results. Now I'm beginning to suspect that it could possibly be the CPU, but it supposedly runs at 2.0 GHz. Are there any other possible variables that could be effecting my CPU or any way of fixing my lag? Please don't suggest temporary fixes like Game Booster (does not help) or to "buy a new computer". I'd like legitimate, helpful responses.