What am i missing....

Alright, i know there are a couple of these floating around and i have read a couple.
My computer that i built like 2months ago is randomly crashing. I willbe doing anything from surfing the web, playing video games, or not doing anything at all and it will basically reboot itself. It goes straight to a black screen for like 1-2sec and then it loads back up.

At first i thought it was my Antec Smart Power 350W power supply. I was upgrading my video card already so i tacked on a Thermaltake purepower 420W.

I added the new power supply and booted up, it has now died like 4-5times. The rails are as followed...

3.3=3.37
5.0=5.13
12=11.97
-12=11.70

I have a chaintech VNF3-250 board, A64 2800+, 512 PC3200 Kingston RAM, 1HDD, 1optical drive, a shittty video card (haven't added the new one yet, that is supposed to be delivered today) 2 120mm fans, 1PCI modem, and the only thing i have for USB is an external HDD.

So i am somewhat sure that its not power supply related. I am almost garenteed that its not heat because my system runs really cool. my PCU is 30idle and never above 58 under load.
So if its not temp, and power....what else could it be?

EDIT: Minor mistakes

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I lost.....again. Atleast it was close....135-70.<P ID="edit"><FONT SIZE=-1><EM>Edited by pickxx on 11/19/04 10:43 AM.</EM></FONT></P>