Murray,
I just got done reading all the posts on this thread.
If your Smoker keeps heating after it gets to the set point, and beyond, it's more than likely the Controller.
The "Swings" a lot of guys are referring to is when the element shuts off at the right time, but the heat keeps rising (coasting). This is obviously not your problem, if the element stays on after it gets to the set point.
I don't think your extension cord will cause your element to stay on past the set point either.
I would write down all the things it does & call
Masterbuilt.
BTW: Do you know which Generation MES you have?? Gen #1, Gen #2, or Gen #2.5?
Here's how you can tell:
Masterbuilt Smokers (Bear's Thoughts & Findings)
MES Generation Number Recognition Pictures & Pics (Digital Units)
Avoid Temp Swings in MES
Bear