Just bought a corsair cmpsu-650tx and installed it. Had the side panel of my case off to make sure everything was working right and noticed that the psu fan wasn't spinning. It would spin for a few seconds when I turned the system on then stop. I thought maybe it had to reach a certain temp before it would turn on or something so I ran furmark. It didn't spin at all first then I looked again after running furmark for like 2 mins and the fan would spin for about 10 seconds then stop for 10 seconds and repeat. I did the benchmark for 15 mins and it did the same. The back of the psu was warm to the touch not hot but warm. Is this normal or should I return it to newegg and get a replacement?
So I guess it's working as intended and PSU's do get warm under load.
EDIT: I just realized that you actually said that the PSU fan is going on and off while under load too. From what I understand, the fan should start spinning once the load gets higher and stop when it goes down again but if it turns off and on while under load then something might be wrong with it.