hey I've got a p4 1.7 ghz w/ 350 watt power supply 1 gb sdram pc 133 300 gb hd, 20 gb hd, ATi x700 Pro 256 mb gddr3 and dvdr drive.

My compuer randomly shuts down esp when running any heavy applications, or even windows media player, it stays off for like 5 min. I can't turn it back on for more than like 3 - 5 seconds after it shuts down and have to wait the 5 min. I thought it was my power supply so i bought a 500 watt power supply and now its doing the same thing over again,
any ideas?