I'm using Cookshack pellets.
No I have not verified with an independent probe yet, but can do that tonight. I think the factory probe cannot be too far off though. When the temp goes down below set point, I do hear the auger then start running longer each 15 second time period as it should do. So I'm thinking the HHT function & control board, are working just fine. I can find nothing obstructing the flue. Alignment of the pellet feed into firepot is good - I've gone thru about 1-1/2 bags of pellets so far and only collected about 5-6 pellets total out of ash dump drawer and nothing from bottom of unit.
My thoughts originally were that the LHT was causing the issue by not feeding enough pellets to keep the fire going. Adjusting the LHT up to 40 did not resolve my issue. It did however prevent the grill from achieving temps under 315* which is understandable with a LHT setting of that high. (Too fast of an idle as Nordy put it in his excellent post on LHT & HHT settings).
Weather in western Washington typically now is around 40-55 degrees, 5-20mph winds, and rain showers. We get lots of little short-lived wind gusts that can be be perfect for blowing smoke back down chimneys or snuffing out a low-burning flame. I keep my grill inside my attached garage when it's not being used, so it's not subject to our moist exterior environment - ie pellets not getting damp/wet.
Regarding the amount of smoke my grill is producing when the temp is doing it's big drop thing and before re-ignition occurs, I can only compare it what my old Amerique would do at start-up when it was over loaded with 5-6 chunks of wood. Imagine that same smoke density (density - not volume) as what comes out of those two vent holes now coming out of my 2-1/4 x 4" flue and it gets to be a bunch of smoke. There is smoke billowing out of the flue, leaking out around the doors as well as both drawers, and out of the pellet hopper. I'm lucky my neighbors have not thought my house was on fire and called the fire department, yet....