AME/PP freeze/crash computer

AMD Ryzen 9 3950X 16-Core Processor 3.49 GHz, 64 GB RAM,  Windows 10 Pro, NVIDIA RTX-3080 GPU

AME V23.6, up to date;    PP 23.5

Media Encoder totally freezes pc at random points while encoding a particular project; also happens when exporting directly from PP. I have to hard-stop the pc and reboot. Have also had pc reboot on its own while encoding the project in AME. When the freeze occurs, I can do nothing at all with the pc, can't even move the mouse cursor or get any response whatsoever from the keyboard.

This is happening with an old project that I'm making changes to. Originally, the project was on the internal drive. Now, it's stored on an external USB drive... I've edited the project on that USB drive, and all seems ok in Premiere. But when I export the project from PP, or encode it in AME, the freeze-up happens after some apparently random period of time. The destination output .mp4 file is to the internal SSD; H.264 mp4, 4K 59.94fps

I have successfully exported partial segments of the 4K project, which is about 1 hour 15 minutes long. But trying to export the entire project, or longer partial segments of it, results in freeze-up and/or crash.