- If you're using a power strip or surge protector, try disconnecting those and connect PC directly to the wall.
- Are you sure those test PSUs were working correctly?
- Examine the cable between front panel and motherboard. Reseat both connectors.
- Bum power button..??
- Is it booting when you unplug/replug power cable because the AC Recovery option is set to "ON" in BIOS setup?
- Is that a work-around? - Connect PC to power strip with its own on/off switch. Set AC Recovery to ON. Shut down normally with Windows and then turn off the strip. Turn strip on when you want it to boot...