What the heck is wrong with my cpu?

I recently picked up a used Dell XPS M1330 but It's got a really weird fault. When under load for any length of time the OS load gets higher and higher (i'm using Rightmark to monitor) and eventually hits 90% . If i stop what i'm doing it slowly goes back down but then it will spike every few seconds or return to 100% and stay there. Even though i shut down the program or task that caused it and the cpu returns to normal it happens again. The only thing i can do is shut down the computer and leave it off for a while.

I thought it was overheating at first as it gets to about 40c when it happens but even when the cpu has cooled it continues to happen. I'm running a clean install of Vista home premium and i've disabled all unnecessary services, aero etc. I have no idea what's going on. How can i establish what's causing this? Has anyone come across an issue like this before? If i can identify the problem i can do something about it. I was going to upgrade the processor but until i know what's going on i can't and will have to return the laptop. I don' think it's an issue with Vista as i've tested a drive with windows 7 and the same thing happens. What could cause performance to degrade over time and then not recover at all, even when the cpu has cooled down? Really weird!

 

That use to happen to me too and then one day my computer would not charge and that was another issue. When dell replaced my motherboard the first time it fixed all of it, but it definitely was an issue

 

Thankyou! I now know there is a fault. In that case i wonder if Dell would fix it. Although reading some of the accounts on here that could take a while! How long did it take to get it replaced? I'll probably take it back though, i'm assuming i'd have to buy a warranty to get any kind of support...

Cheers.