That is very odd. The first time I did a butt, I opened the door each time smoke died out to add more wood (monster mistake). I used 12 ounces of wood total and ruined the meat. But the Amerique always returned to within a couple of degrees of my set temp. I would give the folks as CookShack a call and ask to speak to a tech.
Depending upon the age of your smoker, your element might be going bad. I would check the element in the dark. Set the temp to 225 or 250 or whatever; it should have a pretty uniform glow when heating with no dull or dead spots. No need to run it for more than a few minutes.
You can also check with a loaf pan full of damp sand to simulate a moist hunk of meat and see if your experience was just an odd one off occurrence or if it repeats itself, or is getting worse.
Hope this helps!