My two cents worth. Some folks like ribs that literally fall off the bone with most/all of the fat cooked off, while others like tender ribs but still have the meat firmly attached to the bone. One style takes a longer time than the other.
As for the temperature, there may well be a problem. Then again, one may be inadvertantly checking during the low point of the temperature swing. One last thought is regarding the calibration of the digital temperature probe. Is it accurate when checking the temperature of boiling water?
A second last thought: Take a feeler gauge, brass is flexible, and check for gaps between the door and the box. That could be a source of lost heat. 3M makes a high temperature stainless steel tape that could be trimmed to fit in those areas, like a shim, to seal the gaps. I have had to do that in a couple of areas on my smoker, one for smoke loss and the other for heat escaping.
Last thought. Even with the change out of the controller, has Steve been notified of the continuing problem?