Just my
here. I had similar issues when I got my DB. First cook were two 14 lb briskets. Started at 9 pm and checked at 11 before going to bed. Woke up around 4am and the briskets were already done and temp was at 200. 50 - 75 degree temp swings from left to right. Called up GMG service. They upgraded my controller to a WiFi. Still was having temp swings. I had the old heat shield and I ended up cutting slots in it to allow me to slide it left or right. Still had temp issues and the thermocoupler went bad and was doing burgers and the LED was over 500!! burned the wiring harness. GMG promptly sent me a new controller, thermocoupler and wiring harness. This is was back in 2017 and I still haven't installed the wiring harness as it functions fine. My DB is a 2015 model and sure I was not happy with it but GMG kept working with me on it, even sent me a new heat shield as I was still getting some temp swings yet. I installed a 3" stove pipe elbow on the inside, added a second rack, put a Corian front shelf on it put in two oven therms, thermal blanket, GMG cover, and a small water pan (small loaf pan). It sits outside year round also.
Since my DB was gift, I went directly to GMG Customer Service. They have been great and very helpful. Sorry to hear the issues on your Dad's. I have been very pleased with mine even after all the issues I had. I recommended this to friends and they have actually gotten one. They love it. Temps are very even now, WiFi option is great to use (nice to be able to change the temp right from the bedroom, when doing an overnight cook. I never got to the point of futility with it. Just took my time with it. I was a member on the old Pelletheads site and gathered info there along with here on PF.