There is one other issue that it could be, I had a similar issue but I was fairly confident that it wasn't my PSU, or my MOBO that was my problem. It turned out that the heatsink that I installed (Thermaltake Big Typhoon) was grounding me out. Everything worked perfectly as soon as I fixed the grounding issue. Also with my Mobo I had to have 4 power connector things plugged in for my system to work not just the normal 2.
Anyay I would still probably agree with the common thought here that your PSU is a crapper and u need to toss it and get a new one.