Well, I woke up this afternoon after a nap, and my "main" Q9300 still had its screen active. Which is pretty strange, usually it shuts off. When I got to it, I noticed the mouse wasn't moving either. It had frozen up last night, hard freeze.
So I set the CPU overclock from 3.0 back down to 2.5 (stock) and reduced the vcore to stock. I'm more suspicious it's the new GPU, but ... it's the first troubleshooting step to try, I guess.
I noticed that the GPU in this box got to 70C, whereas, in my other Q9300, it only got to like 63-64C.
In my quad-GPU cruncher, they only get to 43C. (It has a lot more fans, and moves a lot more air.)
However, these cards are rated to 90C, so I don't think that should be a problem.
It could be the OCZ SSD freezing up in the system. Or perhaps the PSU needs changing? It's only 4, maybe 5 years old. It had a 3-year warranty.