Complete system shutdown is a hardware issue, almost 100 percent of the time. I know, had the same problems running games that were using a lot of CPU, system would shut down when the core temp went too high to protect itself. Long story short, bought some thermal paste and a can of compressed air, took my cpu out and reapplied paste (it had ALL dried out), cleaned out the case and fans with the air, haven’t had a crash since. Dust will creep up on you.