I have used 2 psu's:

500watt Antec unit Antec Sonata III Case with 500Watt PSU : ant-son3

580watt Hiper Hiper 580 Watt, ATX v2.2, Temp control Act. PFC, Sli Cert : hyp-4m580

both exhibited the same problems.

So that narrows it further-- not the PSU-- those are both good brands-- with plenty of juice.

I am really leaning towards the MOBO at this point, I'm afraid...
I doubt even power-hungry RAM could cause these symptoms.

I would contact their support-- they will have you try a few things, perhaps-- but I think it's the board-- if it was the Processor it would have never started at all.

Let us know how you make out.

Don't be surprised if it is a bad part. Maybe it is just me, but it seems like manufacturers have been slacking on quality control. I ordered parts for a new rig late December. I had to RMA the ABIT MoBo and then the BFG Tech Video Card. I'm still waiting to put it all together.

Well it was the motherboard. I bought a MSI cheap board and it cured the problem. the P5K boards seem to have quite a few issues. It is also the first time I have bothered with brand name ram. I went out and bought 2x2gig of ram (black dragon) and the aforementioned motherboard. I tried the ram in the P5K board and it worked better but still had the same type of problem. I did some research on the web too and it seems that the current ASUS P5K boards are a bit quirky. I am going to RTM the board and use it as a pvr but there is a lesson here - and that is don't assume a new build is going to work just because it is for sale.