One way to check to see if the problem is heat is to open the case, and run the computer with the case open. If it's better, the problem is likely heat.

Could be a combination of both heat and power supply, since the fan runs off the power supply. Both should be fairly cheap fixes.

I recently had a fan failure. The computer's only a couple of years old. I've had computers that were a lot older, and the fans were fine, but I guess they're not making 'em like they used to. I only had a problem with Poser. Vue was fine, but Poser slowed to a crawl.

You might try going to PCPitstop.com, and running their diagnostics. It's free, and no adware or spyware will be installed (though you may have to drop your firewall temporarily).