Computer Slows down after a couple of hours of use: No more info

Have you completed all of the Troubleshooting steps I gave you? If so you would have a new install without these problems.

Everything apart from the last two steps. I was trying to figure out the cause of the problem so if it every crops up again I know what to do.

It's the worst damn kind of problem, it appears for no good reason and then disapears just as randomly as it started.