Just ran a memtest, seems I have some bad RAM…
Which seems consistent with this Reddit thread: https://reddit.com/r/chia/comments/mjbr6t/plotting_randomly_stopping_without_error_most/
I’m running memtests now on each ram stick to see if it’s one or the other, and already having lots of errors on one stick, so I’ve ordered some different ram to try again tomorrow. Yay.