Hardware issue - been trying to solve for weeks now

Alright, this is a toughie. I work at a PC Repair shop and no one here seems to have any more ideas than what we've already tried. Basically 90% of the time my PC will hang before it gets to the post or at the post. It does not beep, just hangs with all the fans spinning at 100%.

Athlon 64 x2 3800+ (stock speeds)
ASRock 939dual SATA II
2x512 Generic ddr400 memory
X1800XT 256mb
2 WD HDD, 2 optical drives
Antec Truepower 500W (18/17A on 12v)
(I also have a 550W Mushkin b/c I bought it thinking my PS went out, that puts out 20Ax2 on 12V)
Also BIOS settings have all been set to factory settings.

Ive tested the entire hardware system with a P.H.D.v2 hardware diagnostics and everything came back fine without a hitch. Tried swapping out every part except for a motherboard and cpu so far.

I just wanted to get any other opinions before I RMA this board, I'm about to rip it out and put it on the test bench to make sure, but maybe someone else has some ideas I could try.
Thanks a lot

It may be your power supply. I've lost count of the times I have seen a workstation with a failing (but not dead yet) power supply turn on but fail to post (just a blank screen, no beep errors). About 95% of the time with those, changing the power supply is the solution.

If you have or can find a spare power supply, just pop it in and see if she goes.

-dolynick

Apologies, I completely missed the stating that you'd tried a new PSU already in your original post. My bad.

If it's working torn down on the bench then that is an odd one. You may be right that its a short when in the case, touching somewhere or something like that. Unusual, but possible.

Another possibility is that you didn't have something seated right when it was all inside the case, though one would think you'd get some kind of beep error.

I would rebuild it in the case again and just see what happens.

-dolynick

to follow up on what someone said, i had a comptuer what we built outside of a computer case and everything worked fine, i didn't think twice about it, so i slapped everything in a case and let someone have it, well they called me back saying, "All i hear are the fans, nothing happens", upon getting the computer back i tried everything you mentiond above, i didn't know what else to do, well needless to say, it was a short in the case, everytime the motherboard was in there some of the pins would touch the case causing it not to do anything, i fixed this by taking a very thin piece of packing material and placing underneath the motherboard while i was screwing it in, this fixed everything, comptuer now works fine. See if that helps.

Yea, i used a the foam that was with the motherboard when i purchased it, i just put it under the board and screwed it in, no problems so far from it.