Theo,
Are you using PCDR 6 or 7? Depending on the post card type you may have to change the card from normal to passive or the I/O address. This is done by dip switches located on the card. The instructions for your card are included in the package.
I would unhook everything, SATA drives, any expansion cards and see if you get post codes. It could be a card locking up the bus.
I would also try a different video card, both a PCIe and an older PCI card.
Taking another look at the Power Supply may be in order. Even if the voltages are correct they can fall off under load.
-Fred