W2k Shuts Down

I think I am having a problem with hibernate on a W2kSP4 computer w/all security patches. Mcafee Virus Scan and Zonelab Pro.
Sometimes it shuts down and when you try to start it again it won’t. I have to unplug it for a few minutes and then hitting the power button will start it up.
It is a tyan tiger 133 (S1834) motherboard with 512MB RAM 45Mhz PII processor.
I thought I had a problem with cooling or power so I opened the case for more cooling and then bought a new Sonata 380Watt power supply, no relief.
I replaced the SCSI card, two Scsi hard drives and two SCSI (CDR & CDROM)s with one IDE ATA card with a new IDE drive and a Sony CDR.
computer also has an ISA Creative Soundbalster, PCI 3COM NIC, Adaptec PCI USB card, with an AGP 64MB nVidea Video card.
I welcome all ideas.