I have a dell 780 sff tower, service tag 81dnk4j that won't power up. The green power light comes on momentarily when pressed but the device doesn't so anything, and then there's two LED's flashing together at the bottom front of the case. What's strange is if I press and hold the small button on the psu at the back I get a green light and the fan starts going but the machine goes dead again when I release this button. If I press and hold this rear button again and then press the front power button the dell powers up properly and works. What would this fault point to?