System over a year old ? Open case, blow out dust from heatsink on CPU and on video card.
Check temps with HWMONITOR. (google it, free download) anything approaching 100c ?
Open case, point a big house fan at the inside of the case and turn on high. If the restart problem goes away suspect you have temp issues on part of the video card or MB or PSU.
Assume if you restart rather than shutdown/reboot you do not have a video problem ?
Reseat power cables and video card.
GL