Sounds like its over heating to me.
When was the last time you cleaned off the dust on the CPU fan or checked to see if its even working?

Turn the computer off, open the case, use a Q-tip & clean off the fan blades. Turn the computer with the case still open & see if the fan startes to move..

Unless you have a $$$ case, the CPU seems to over heat if the fan get dirty. We usually just left the case open & pointed a large fan blowing into the case. We now have 80-100$ cases & they keep the computer really cool due to LOTS of fans, larger fans & more empty space.



Remember if your CPU overheats too many times, it'll jusr fry!!! SO its best to clean it every 6 months.



EDIT: if you smoke or others smoke near the computer, clean the fan monthly, the tar is heavy on the fan & REALLY bogs it down.