Ive seen this many many times,,
Some power supplies have pins in the AC power connector that are smaller "skinner" than others.
If you use a different power cable than the one that comes with it, it will be loose.
What I do to correct this is to use pliers to give a slight twist to the pins inside the power connector on the power supply.
This problem has been resolved, thanks a lot for your help it allowed me to rule out heaps of possibilities.
You wouldn't believe what caused it - A dodgy reset button on the case. The case is a $300 Thermaltake level 10 gt. I noticed that whenever I moved the case the system would restart, this had me so puzzled that I literally fault found every bit of the pc, unconnecting everything and starting it up then moving my system to see if it would restart, eventually I looked at the front of my case and noticed the reset button is broken and has been permanently pressed in.
Long story shot I disconnected the reset switch from the motherboard and my system never restarts when I move the case again. The problem would only occur whenever I moved the case and not when it was still, so the slightest movement were somehow enough to engage the reset button.