I have just bought my second one of these PSU's, and had this problem for around 2 weeks before having time to spend on fixing it.
When I got around to grabbing my old PSU out of the old system, I noticed I had padded the top left mounting screw with fibre washers and it all came flooding back..
2 years ago I had the same problem, random resets without warning after hours of perfectly normal use (heavy or not), then repeated resets before BIOS Post. I found that I had used standard case screws to mount the PSU to the backplate of the case, and one of these had reached far enough in to touch the main PCB. On padding this screw out all the problems went away.
The mounting hole at fault is the top left hole, when seen from the rear, with the cooling fan port facing down. Hope this helps anyone else who might get this problem, and might shed light on different rated versions of this PSU.
After padding the screw I am able to run either machine (Asus maximus VI Impact + Corsair Graphite 360T mini-ITX case vs. Asus z87 pro + Coolermaster HAF XB mATX) with either PSU with no problems.
