Powering On Problem

I had an Antec 400 True psu that died on me. So I decided to replace it with
the Antec Trueblue 480. After installing it, when I would start up my
computer, it would turn on for a couple seconds, then the power would go off
completely for about 5 seconds, then the computer would power on normally.
This didn't happen every single time the computer started, but often enough
that I thought I had another bad psu. So I took the Antec back and got a
Thermaltake Purepower 560. Everything was fine for a few days but then this
morning when I turned on my computer, the same thing happened wherein I would
power up, within a couple seconds the computer would power off for about 5
seconds, then power up fine. So...this leads me to believe it was never the
psu but something else. My thoughts are going to the Soyo mobo. Does anyone
have any thoughts on this? Thanks in advance for any help. Below are my
specs:
P4 3.2C on Soyo SY-P4I865PE Plus Dragon 2 v. 1.0
Corsair TWINX1024-3200XL (2X512)
BFG GeForce 5900 Ultra
Audigy soundcard
WD SE 80G hdd w/8 mb cache
BenQ DVD Writer, CDRW & Lite On DVD Rom
Thermaltake Purepower 560 psu
WinXP Pro - SP1