Usually random power cuts and restarts have to do with the PSU. Its a new unit from a good brand so it being broke isnt a huge possibility, but it might be.
Pop open the case and check if there are any power cables (particularly the 24pin ATX and 4pin CPU cables) that are barely plugged in.
Check if the power cable is plugged firmly into the PSU and the wall.
Check to see if anything is shorting out inside the case. This could mean you left extra standoffs behind the mobo, dont have standoffs outright or something is bridging the mobo to the case.
If none of that succeeds, may want to extract all the components out of the case and start breadboarding. Basically assemble to components on a non-conductive surface (a wooden table, mobo box) and see if it works. If it doesnt here, you can be fairly sure its a hardware fault.