It sounds like you PC is over heating...
With the power diconnected Get a can of compressed air ( Maplins, Tandy's Etc) and blow out all the dust thats built up inside your PC.. Pay Attention to anything with a Heatsink and/or a fan. Make sure you clean all including the power supply.
IMPORTANT DO NOT HAVE THE PC PLUGGED INTO THE MAINS
Try this it should help
Martin
May 2007