I had a similar problem on my old athlon xp machine. After checking temps I found it was that the graphics card was getting up to about 70 degrees, which was filling the case with warm air and thus forcing the cpu temp up. I put a zalman fan on the graphcs card and problem solved.

Simple test is to try running it with the case open, that improves airflow and cooling, if it works with the case open you've got a cooling problem.

Check temps and upgrade cooling as required.