Time to do some troubleshooting testing of the oven. First check is the accuracy of the thermocouple. I cross-checked it against a fluke type K thermocouple plugged into my meter. Looks accurate enough...at this temperature at least.

Next was to check the responsiveness. A quick test comparing the oven thermocouple to the Fluke one shows the oven thermocouple is much less responsive. It's to be expected, I guess as it is a much heavier gauge wire than the one plugged into the Fluke.

OK, now with the meter connected to the rear thermocouple, and heating the oven, it seems the rear of the oven can be as much as 100 C hotter than at the front. That explains why the ends of parts sticking in the oven have been experiencing blisters. It's much hotter back there. Very informative test. Now, what to do about it?