Since the thread was originally about diagnosing this lockup, let me work on next steps. So far thank you @guttorm for telling me about stress and about lm_sensors.

1) If there was a lockup from the temp running too hot - would an entry appear anywhere in one of the system logs? Maybe a warning right before it went so hot the system froze?

2) I'll have to watch the temps to see for comparison purposes, but so far it's been OK rendering when the source files are from OBS and it's locked up on files from my Canon Rebel T6s. Any idea on whether different source files would cause a different load on the render process? I saw a thread on Kdenlive's forums that someone had a different Canon camera and while the end result wasn't a system lockup, the Kdenlive devs did recommend transcoding his clips to DXDC or something like that.

3) Are there kernel or ffmpeg or mlt changes that would cause the CPU to run hotter now than it used to? Sure, it's possible that the room the computer's in is just hotter because the lack of heat outside means we're not running the A/C, but I was able to render files from the Canon this summer.

4) If it were overheating, shouldn't the computer be shutting off or rebooting rather than freezing?