Computer Powers Off Using Rocksmith

Hi!

I have a new (to me) refurbished Dell Optiplex 9020 running Win10 Pro, build 19042. Core i7 at 3.26 GHz.
16.0 GB Ram
I installed a GeForce GTX 660 we had used on an older computer.
That required a new power supply so I bought a 750W ATX supply from an eBay seller and picked up the adapters to hook everything up.

Everything seems to work quite well. I have no problems running anything so far, including CAD software.

Except that I like to play Rocksmith2014 on my Steam account. That requires hooking up a standard 6-string electric guitar through a special USB adapter cable. (I've used this flawlessly on several computers since 2015.)

Steam and Rocksmith2014 all work beautifully except that quite often, and very suddenly, the computer will power off in the middle of a song. Sometimes after playing a while, sometimes just a few notes in.

I've tried using the USB ports on the front of the tower, and the set on the back. I've tried two different Rocksmith cables with two different guitars (and numerous songs!). Doesn't seem to make any difference. A few bars into a song, "pop" it's just OFF.

I've read a few things on the Steam forums about overheating issues so I did download OpenHarwareMonitor and looked at the various temps I'm getting. Sometimes around 60C, but not much more while playing. I've tried leaving the side cover off to see if more air would keep it alive but no luck.

So I downloaded FurMark video stress test program and ran it. I saw temps steady at over 80C but the machine never shut down for two hours before I decided to let it take a rest.

So I don't think overheating is the problem.

How else do I go about isolating what's causing this shut off?

Thanks!

 

So, what would be some good ways to start trying to isolate the problem? Does this sound like a power supply failure? Could I have gotten a bad one? Or a short circuit some where? Are there circuit breakers (or the software equivalent) which would kill power if something shorts? Like maybe in the USB connections?

Any ideas at all?