This is my opinion on what happens with the gasket. At settings below 300. Not enough heat escapes. The temp goes well over set point. And the controller does all it can to reduce the temp. By the time the temp drops down to set point, the fire is about out. So it drops well below set point. At which point the controller keeps sending a lot of pellets trying to recover the temp. Now it has lots of pellets and spikes even higher over the set point, and takes even longer to come back down. If it takes too long to come back down to set point, the fire is out.
The Smokedaddy controller does seem to handle this situation better. I suspect because it also controls the fan speed. It appears to still feed some pellets in but pulses the fan slowly while reducing the temp. But this reduces the cycling that produces the smoke.
A simple way to check if this is your problem, is to shim the door open a touch. I use my smoke tube between the front shelf and the handle on the lid. All you need is the thickness of one or two coins. Easy and does not cost a thing to try.