I haven't been following the entire thread but from what I've read it does sound like the PSU could be the culprit.
The hard resets during only certain games sounds a lot like an issue I had few years ago with an old Corsair HX750. Everything with my system worked fine except for when playing a certain game, Dragon's Dogma. Only that game was hard resetting the system. For quite a while wasted time troubleshooting with no success figuring it out, until I eventually deducted it was the PSU. Made an RMA to Corsair describing the problem and they replaced it. I bought a different PSU but once replaced the problem was resolved.
Failing PSU's can be tough to identify because like Nunz said they get worse over time. They don't always just die suddenly.