I work in the IT Support field, assembled PC's since early 2005 without any issues so the quality of the build is not questionable.
Alright, an update.
I changed my PSU to a Corsair TX750M but got restarts once more, this time after each boot it tells me that wattman got restored due to an unexpected shutdown, indicated a video card crash. I switched my PCIE from 4 to 3 in BIOS, looks very stable so far. No performance lost.