Diablotec isnt like a good brand PSU. You'd be lucky if its really a 400w unit.
Replace PSU is what I would try.
Before i had a 450 watt pws and it worked fine on the old motherboard, but when i got the z77 motherboard it restarts a few sec in game. So i upgraded to a 600 watts one, pretty sure pws isnt the problem i also tried using my brothers 600 watts and it still has the same problem.