Print failure because of nozzle temperature error. Diagnose nozzle. PID tuning. Everything looks fine. I thought I had PID tuned but I must not have. Print a part. Print failure because of nozzle temperature error. Ugh, here we go again. Wash, rinse, repeat.

It turned out to be that my extruder JST connector was loose in the printhead. It was snug enough that the nozzle would be hold its temperature stable while it was sitting still. But, as soon as it started moving the connector would become disconnected at times and create problems for the nozzle to hold its temperature steady. What a pain.

Simple fix, though. Just push the connector back in place. Make checking your connections a part of your monthly maintenance routine.