I have had my 34 since Jan of this 2020 mine has done the same thing. I called them the first and they sent out a new controller. I put that in with no better results. Called them again and was on the phone with the guy for at least 45mins, very nice super helpful. He sent me out a new lid and hinges. It still has a 12 to 15 degree variance but nothing like it was. I can say make sure you clean the fire can and belly after every third or fourth smoke. I have been thinking about getting a ceramic seal for mine but haven’t decided yetI'm not sure whether it's normal or not, but I have a similar issue with my 34" Pro. I have done five cooks with mine in the last 6 weeks and the temp was erratic during all five cooks.
For instance, if the temp is set at 250, the temp will rise to the 265-270 range and then fall to the 225-235 range throughout the cook. The ambient temperature for all of these cooks was in the 68-82 degree range and I was using new, dry Traeger branded pellets.
I have been using offset and barrel style smokers for many years and am used to some fluctuation. I guess I was expecting the temps to be a bit more steady with the Traeger. I would be very interested in hearing from others if these temperature swings are normal. While I am somewhat disappointed in the erratic temps, I have been happy with the overall results I have achieved so far.