You might try, if you have a volt meter, see if the +12v and +5v rail off the PSU are actually putting out the correct voltage.
If not then there's a definite possibility that the PSU is bad.
It could also be that the pins inside the connector aren't connecting right. I know it may sound stupid, but have you checked to make sure that the molex plugs are not allowing the pins inside to slide back when you connect it?
There could also be a solder connection that has worked itself loose.
Just some suggestions
Mind Scream out