I had the same issue. I have the PB550 and it has a sliding draft damper on the back at the top instead of a smoke stack. I set the temp for 300F and let it come up to its temp. When it maxed out, I opened the damper about 1/4 inch and watched the temp for 20 minutes. After doing this 2 or 3 times, I had to close it back about 1/16 inch to get the actual oven temp to a near match with the temp setting. It varied over and under 14 degrees than the set temp, and that was close enough not to spend three hours adjusting the damper fighting with steel and heat heat dynamics. Its 12 guqage steel so its not going to hold temp like 10 or 7 guage steel, and its an oven not a rocket, so i settled for the improvement. If you dont have the sliding damper, try adjusting the lid on your stack until you get close enough to the set temp. its nor a Rectec so it will have a 10 -20 degree fluctuation from the set temp. If its too hot open the vent if its not hot enough close the vent a little at a time and watch the temp.
I am thinking the issue was the draft because this is a forced draft oven. It has a force draft fan without an induced draft fan, so the air is forced in but not induced out. If it had an induced draft fan in the stack, it would have a balanced draft.
