my first thought would be the power supply, size seems fine, maybe its not working properly. Check all your jumpers ect..
take out every expansion card possible, so that you have only whats neccessary to run the system unit. One stick of ram in if you have more than that, etc.....
If it boots stable, then reinstall expansion cards one at a time until you find the problem. It could be a improperly installed card, or not pushed all the way in. I think you normally get a CPU error if the cpu is bad, but thats not always.
From what your describing, really sounds like the psu.
Does it POST?