These are quite often heat-related problems - try taking the side off your PC and see if it runs any better - if so; have a look at the fans inside. If any aren't spinning - replace immediately.
Also; power off and look for dust in and around the fans (and elsewhere in there!) - get a can of compressed air from your local PC computer store and give it a good blast if it's particularly dusty (don't use a hoover or an air-line as this will almost certainly lose components off your PC!)
Best of luck