Frequent Crashing

Hi Everyone,

I am experiencing frequent crashing on my new PC and am having trouble to determine the cause.  Windows is Event Log is not showing anything at the time of the crash, and I only see Power Kernel errors which appear after I have already restarted.  (When the crashes occur, I am unable to ctrl alt delete or alt f4 or win+shft+ctrl+b to reset the graphics driver - the only thing I can do is hold down the shutdown button).  

I've had the PC for 2 weeks and use it mainly for gaming.  In my first week, I was experiencing frequent crashing in games (usually crashing within 10 minutes - 3 hours of straight playing).  I couldn't figure out what caused it but decided on switching my RAM from EXPO 2 to EXPO 1 profile.

This seemed to have fixed the issue for a week, but just tonight I experienced another crash 2 hours into gaming.  This time upon trying to restart, Windows kept on crashing while I attempted to login.  I was able to reach the recovery environment and start in safe mode.  After restarting, I am now back in normal mode and able to do general browsing. (Have also disabled EXPO entirely).  

Given that I was having trouble getting into Windows again, is it possible that my issue might be related to Windows and any suggestions what I can do to find the cause /fix this?  

Other things I have already tried:

Running Prime 95/Furmark stress tests on CPU/GPU (individually and simultaneously for 30 minutes) with no crash.

Memtest 86 4 passes did not turn up any errors (while I still had EXPO 1 enabled).