Yep - did a re-calibration from actual room temp and the values are back to fully stable.
Also, I removed the housing from the effector thermistor, looking for any issues, and the crimp was clean. The wire is pretty small so maybe a good crimp wasn't after 2 years of shaking. I went ahead and soldered it just in case, and I haven't had any temperature errors in about 5 hrs now. It's not proof that I've fixed the issue, but it's a good sign! About the only thing left as a possible source would be the thermistor wiring atop the effector that connects to the Duet.