Is the air coming from the rear fan cold, warm, hot, or very hot?
Open the box and try to figure out what is getting hot? CPU, RAM,
plugin cards, power supply, etc.
Make sure the CPU is properly seated and that the heat sink is
properly attached to the CPU. You symptoms sound very much like an
overheated CPU.
Look at the electrolytic capacitors surrounding the CPU and see if any
of them look scorched, melted, leaking brown goo, or have the tops
bulging. If that's the case, see:
<http://en.wikipedia.org/wiki/Capacitor_plague>
<http://www.badcaps.net>
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com je...@cruzio.com
# http://www.LearnByDestroying.com AE6KS
Check the cooling arrangements, particularly the processor heatsink/fan
combo. Check the fan is working correctly and the heatsink fins are
clear of dust. Then have a look that the clamping mechanism is holding
the heatsink tight to the top of the processor. While you've got the
heatsink off, clean off any gunk and add new thermal paste.
Finally check all fans are working and there are no obstructed vents.
It might help, it sounds like an overheating issue. Is it overclocked?