Get a tool that logs all your voltages, fe. speedfan, and then stresstest some heavy 3d programs and watch how much your 12v line drops. If it's your PSU the 12v line should drop considerably.
Check for GPU artifacts too with whatever tool u prefer and if those two don't net results I would start swapping parts (GPU,RAM,HDD,CPU,motherboard..preferrably in that order) till u can isolate the cause.
Last step ofcourse requires tried and tested parts that work on the same sockets as your current gear.