is it old?
Normally these symptoms indicate overheating in which case it will automatically shut down when it gets too hot it will vary when it shuts down in hot or cold weather etc.
Reasons for overheating are
1. fan isnt working or sluggish broken fan blade clogged up
with dust and crap try to have a look at it
and remove any dust and fluff if its there.
theres two fans at least usually one on chip on the motherboard and one at the back in the power supply
2 your comp is all blocked in and the air cant escape bad ventilation
3 its overclocked and it needs to run at a slower speed.
4 some motherboard cpu combinations do run hot anyway take the side of the case off
5 theres probably a lot more but theyre the most common
Ps this is only relates if its overheating it might be something totally different
Mark
August 2005