Also, check to ensure there are no "extra" support posts installed in the case under the motherboard.
When upgrading to a new motherboard once, I hadn't noticed that the new board did not require one of the centrally located posts which was used by the old board. Turns out that extra post sat directly on exposed circuitry and caused a short-to-ground which kept the board from powering up.
I had a similar problem with my asus p5q pro although it wasnt a ''extra'' posts that was installed, but apparently it was making it unable to boot
my solution was.. remove the specific screw that went in it. Took me 2 days to figure it out lol.. been running smooth since a year ago