start by running memtest when you go off to sleep. i see if one of the sticks went bad. if it not the memory stick then it might be the gpu or the ram on the video card. I would pop the case and see that there no dust clogging any of the fans and the fans are moving. i get a copy of fanspeed or hardware monitor and watch the gpu temps. check that the fan still spinning up to speed as the video card heats up. if memtest passes i run a gpu burn in test to see if stressing out the video card cause the issue to pop back up.