So, good news and bad news - after turning the resolution down from 2560x1440 to 1920x1080 and also turning the graphics settings down from Maximum to High (Desktop) I was able to play for a solid 3+ hours with no crashes! The bad news is, boy I sure hate to have to play on those settings after spending all that money on a nice new rig. XD

MiniTofu - It is a desktop, and I opened it up and gave it a good cleaning to be sure it wasn't dust and cat hair causing the issue, so I should be good on that front. Thanks for the suggestion though!

worldofneil, MilkieTea - I've seen this suggested a few other places as well, for folks with similar shutdown issues, so I'm going to look into getting a new power supply to try that out next - thank you!