Computer shutting down

[sini00]

Here's my specs:
AMD 64 2800+ Athlon processor
Asus K8N motherboard
GeoForce 6600 graphics card
OS - XP
1gig ram pc3200
450 watt power supply

Here's what's happening:
The computer will shut down regularly even if I am just browsing the internet. If I immediately power back up it will work. If it sits for a period of time I have to push start button upward of 100 times before it will completely boot up.

What I've tried:
Checked power supply - showed within specs
New hard drive
swapped ram around to different slots

Any ideas what else I can do?

 

[genec57]

You need to substitute the PSU, they will pass checks and still be bad. If not that, most likely to be motherboard.