Getting green boxes on monitor, then pc restarts.

I have Asus Gtx 650Ti Boost, after 4-6 hours of playing game or browsing internet, on my monitor i get green boxes all over the screen then pc restarts, and after restart pc boots normal without lines, one day after restart i let pc works for about 12 hours, but everything is normal. I replaced the thermal paste, and replacing thermal paste didn’t help.Here is some pic of problem