Built a new PC, now suddenly when I try to run memtest/prime 95/and "real" game, it reboots itself.

Here are the specs to start with:

When I originally built this system a few weeks back, I was getting random BSOD errors, through some troubleshooting I was informed that the problem was incompatible RAM. Replaced it with recommended RAM, reseated the CPU, and everything was running smoothly for a week or so. I was able to run Prime 95 overnight without issue, and passed memtest no problem.

Now today the person the PC was built for returns, and plays some games on it for a bit. TF2 and some other lighter stuff. Then we begin to play Titan Quest together and she says the game is acting "jumpy". I blame it on the onboard video, and she continues to play, then the computer reboots itself randomly. Now, anytime I try to run anything harsh (Titan Quest, Prime 95, the computer reboots itself. The same thing occurs when I try to run memtest before starting windows. I have no idea where to go from here. I have tested the computer with a different PSU as that's what google led me to, but that changed nothing. I'd greatly appreciate any help or advice that can be offered. Thank you.