>>It's only a few weeks old
You have a brand new computer then? Or are you saying you bought a used computer that you have only had a few weeks? If it's brand new, you need to call technical support of the brand (Dell, HP, etc) you have because you have a Warranty. Tinkering with it will void your warranty.
I've seen these symptoms with a hard drive that was going bad, with a motherboard going bad, with a video card issue. It could be just about anything. You need to state exactly what you are doing when you see this. What kind of functions are you performing on the computer when this happens? Games? Music playing? Internet surfing? Using some other software? If your MOBO, video card, or hard drive aren't going bad, then generally when you see symptoms of slow down they are related to something you're doing that's taxing the computer's resources.