Thimgs you might try.... if you have not already.

Open the case and vacumn all the dust out.

Take out the memory card(s). clean and reseat, following careful static and handling guidlines.

Use a Registry cleaner/drefragger, most problems start here.

Generally problems arise over time, so defrag the harddrive The use of a wonderful utility like Tune Up Utlilities, and if it comes to it, complete format of the drive and reinstal (after backing up important files first.

Bets of luck.