Gin,
Get yourself a couple of cans of canned air, any computer or office supply store should have them.
Check your CPU heat sink. It may have some of the fins clogged with dust. I have seen systems where the fan packs dust between them so as to restrict airflow.
Make sure the heat sink is seated on the CPU properly, that all 4 posts are firmly attached to the motherboard. I have seen systems where the heat sink has partially detached itself causing incomplete contact with the CPU.
Check the ventilation holes in the case, dust bunnies can accumulate and cause airflow problems. Blow out everything well.
If none of the above works purchase some quality thermal compound from a computer store. Carefully remove the heat sink / fan assembly from the motherboard. Scrape the old compound from the heat sink and CPU with a utility razor blade and wipe clean with a tissue. Reapply new compound to the CPU and reattach the heat sink.
If there is something mechanically wrong with the heat sink mounting you could purchase an aftermarket / high performance heat sink and follow the above steps.
Please let us know what you find.
-Fred