I talked to tech support at Pit Boss and his first words was "he had never heard of this doing that".  After I described that this was the first time being used for a long smoke he suggested to open the chimney cap more.

This seems to be the consensus here to do that also especially with it not getting enough air to run the smoke tube.

It sounds like it went out, dumped more pellets and relit!  Take the chimney off!

Mine does the same thing and I have the cap as high as possible without taking it off. It ran for 2 hours at 250 without issue that I noticed. I turned it down to 200 and a couple hours later it dumped excess fuel. The temp dropped from 200 to 155, billowed nasty smoke, then I heard the fire start blazing and the temp went up to 260. It stays within 20 degrees of the set point except when it does this. It is definitely an issue with an excess of fuel being dumped on a small fire. I am trying 225 now and see if I have the issue again. I am not sure what, or if anything can be done to correct it. I'll probably call the manufacturer in the morning. For the record, ambient conditions are right at 100 degrees with 10 mph winds with 20 mph gusts and 38% humidity.

I am not the OP but I am having similar issues.

Last night I said I was going to try at 225 and about 2 hours after I dropped the temperature, I looked out and saw the temp at 300 and watched it climb to 335 before coming back down. I missed the billowing smoke part this time though. Normally mine runs at set temperature +/- 20 degrees which I find perfectly acceptable. I have actually witnessed the issue a few times. In my case the tipping point is when the valley of the temp swing hits about 20 degrees below the set temperature. At that point the temperature usually starts climbing back up. When it does not, it starts to make the nasty smoke and will drop another 20 to 30 degrees and then I can hear the fire roar to life and it will shoot as much as 110 to 120 degrees over the set temperature. This is with the cap as open as possible. There are only a few threads meshing with the nut, just enough to keep it from falling over.

As a side note it seems to do this when the temperature rise over ambient is less than around 150 degrees. I am experimenting today. I am starting at 225 and if I have the issue I will turn it up to 250, then 275, until it does not happen.

What level do you have the water pan on?  I had mine at the lowest level which was directly above the firepot heat deflector.

I am on a Facebook group for PitBoss owners and several of them are having the same issues.  Some say it is the 225 temperature setting and some say it is a bad thermocouple.

My other times I was running with water pan empty and at the lowest location. All of the temp I spoke before were what the controller displayed. For my experiment yesterday I raised the water pan to the second lowest location, filled it about 3/4 of the way full and removed the chimney cap. I ran it for at least a couple hours each at 225, then 200, then 175, and never had the issue at all that I noticed. I ran it for about 8 hous in all and the water pan was a 1/4 of the way full at the end. There were a couple times the temp display on the controller would go about 50 over the set temp but the cooking cavity was only about 30 or 35 over. The important thing was I did not see the nasty smoke all afternoon. If I can spend another 8 hour period in the 225 range without the issue I will be happy.

For the record, it was another 100 degree day here in South Texas.

I plan on doing some testing myself in a few days.  I will go ahead and move the water pan up one spot.  I also want to see if my smoke tube would stay lit.  I will take the chimney cap completely off like Slaga did.

It finally quite raining for a little while and I tested this to see if the smoke tube would stay lit.  It would not.  I had the temperature set on 200 degrees with the smoke stack cap completely off.  It must have to do with not getting any air from the bottom.  I had it about 6" above the lowest point and the water pan.

I am smoking some Jimmy Dean breakfast sausage for a get-together tonight.  After a hour and a half the temperature is staying pretty steady.

I think that door seal is too tight and there is only a fan for the fire pot and not one to circulate air in the unit so it probably is not enough to keep that fire going in the tube.  Hopefully, it still works out well for you on your cooks without it.  I have yet to fix mine.  Just haven't had the desire to get it done yet.

I used an Amazn tray with bust lit at both ends and the middle set on top of the deflector to smoke some cheese, but I did not have the unit on. I would try putting the tube on top of the louvers on the water pan where you have airflow. I know in my RT I pu it along the back wall with the air coming up from the bottom, it stayed lit.

Besides the smoke tube not working the smoked breakfast sausage was a big hit.  I only had the smoke tube in there to see if it would work with the chimney cap off.  I never had any flare-ups like I did before.

If you really think  you need to use a smoke tube, try to pry the door open slightly to let more air in.

My other times I was running with water pan empty and at the lowest location. All of the temp I spoke before were what the controller displayed. For my experiment yesterday I raised the water pan to the second lowest location, filled it about 3/4 of the way full and removed the chimney cap. I ran it for at least a couple hours each at 225, then 200, then 175, and never had the issue at all that I noticed. I ran it for about 8 hous in all and the water pan was a 1/4 of the way full at the end. There were a couple times the temp display on the controller would go about 50 over the set temp but the cooking cavity was only about 30 or 35 over. The important thing was I did not see the nasty smoke all afternoon. If I can spend another 8 hour period in the 225 range without the issue I will be happy.

For the record, it was another 100 degree day here in South Texas.

I just wanted to update this. I did a 10-hour jerky cook yesterday and I started with an 80% full water pan and never had the issue at all. I really think having water in the pan (which the manufacturer specifically stated in the manual) is a must. Mine is running great when I have water in the pan like I am supposed to.

I am not sure I did every time in the Product Review, but I think I did 85% of the time, and I do not remember having any issues!

The water pan is a heatsink only with water or an other liquid in it... Working, it absorbs or gives off heat depending on the temperature differential of the pit and pan.

I think there is a little more to it then heat sink...what about the evaporating water dynamic?