I had a similar problem with my DLX, only worse. I had severe swings sometimes over 100F higher than setpoint. I was never able to get through an overnight cook without a flameout. CC was very accommodating and send replacement parts and pellets but none of it resolved the issue. In the end after a few months they were nice enough to work with online seller and arrange for a return. I never was able to figure out for sure what the problem was but when I was disassembling it for return I noticed what look like a crack in the air channel. And of all of the replacement parts they sent, they did not include the main blower fan. So one possibility was an airflow problem caused by either a leak or weak fan. Months after returning it some other members on PH mentioned they had the same problem after installing a door gasket - and after they removed the gasket the grill functioned well again. I had installed a gasket, but no longer had the grill to test that theory. There have been a few others reporting similar problems with no gasket installed. Still plenty of more folks that have zero problems either with or without.
The problem you are having is almost certainly not the controller or RTD. Unless things have changed drastically, CC uses a very simple controller algorithm that controls only the auger duty cycle. Below set point the auger has one fixed duty cycle with a longer 'on' time , and above set point it has a duty cycle with a shorter 'on' time. That's it. When CC sent me the replacement controller and RTD I already knew it wasn't going to fix my problem because the probe was reading correctly compared to a 3rd party therm, and I had already verified the duty cycle was correct based on other members data.
So if it's not the electronics, what does that leave? It's gotta be an airflow problem. Maybe the fan is under powered which causes problems if there is a small leak, or if there is more back pressure than expected (gasket??). There's something out of whack, and the controller is not sophisticated enough to deal with it.
So, I think what I would try is prop the door open slightly and see if that changes anything.
Also, regarding 'Smoke' modes, IIRC, those are timed cycle modes only - that is, they have a fixed duty cycle that does use the RTD as input.