Do you have all your drivers up-to-date? Newest version of DirectX? Have you looked at your temps? There could be quite a few things. Also, check your settings make sure you're not overclocking if your computer can't handle it.
I think its the PSU, if the PSU is not giving up good power to the Graphics card or the CPU then the PC has stuttering. I had this problem in the past, fixed it by getting a high end PSU Anyways, my opinion would be to go to a technician and ask him.