I have a PC Power and Cooling T51ASL ATX P/S (24 +8 pin) and have the same problems with not being able to do a cold reboot. I don't have an OS installed as I was trying to configure the bios due to my case not having a reset switch. I wanted to make sure everything worked for power on and power off.
After manually turning the power off with the switch it will not power back up, unless I turn the P/S power switch off for a moment and then back on. I was able to pull the ATX switch from the header on the MB and use the built in switch to turn it on and off and the reset button worked as well for a majority of my testing. When I hooked the case switch back up to the ATX header I had the same problems.
I did notice from time to time after resetting the P/S switch and draining the MB, that when I turned it back on it would give 3 small beeps. Not certain what that means as it didn't sound like a normal post test error.
I have tried plugging the FD power connector on the board as someone suggested but it did not help. (Only one GPU so I didn't think that would matter) I also flashed the bios to 12/7/05 and that didn't fix it.
I have also played with every bios setting related ACPI, ATX switch, wake on power & mouse, and even played with the 5VSB jumpers on the board for both USB and PS/2. (Back to default 5V now)
Any clues? Please Help!