PC Probs - Faulty PSU?

Been having constant hassles with my PC, but I've kinda isolated where the problem is.

My PC would keep resetting intermittently. After some time, I've ruled out everything except PSU or motherboard. I just bought a new PSU (480W) to replace my existing one (550W). Whilst doing this, I noticed that on the old PSU's connection to the motherboard, some of the casing around the pins has become scorched and black. Not a great sign.

So I put the new PSU in, and nothing happens. Well - not nothing - but on releasing the power button, the fans almost move for about .5 second and the red LED appears on the motherboard to indicate that there is power. That's where it stops.

Put the old PSU back in, and it powers just fine (but obviously have the original resetting problem).

Put the new PSU onto a different PC, and it powers up fine too.

So, what am I missing? Both PSUs seem to work - albeit the old one is faulty. But on my PC, the newer one fails to power anything for more than half a second if that. I havent even connected it to anything other than the motherboard just to see if it power up.

Grr.

Motherboard: EPoX 8K7A
CPU: AMD XP2200+
Old PSU: CWT-550ADP (550w)
New PSU: Colors 400U-GSPA (480w)