I fought almost same problem all last winter, cleaned everything multiple times, checked all gaskets fixing those that needed it, played with adjustments and stove always seemed to run bit dirty. All without success, finally gave up and just ran it at 6 or lower.
This year no problems until stove running on 8 for several hours (looking for high heat to quickly warm building I only use few hours each day & weekends)and then usually time to back it down anyway. Differences being: somewhat colder outside this year and brand of pellets I'm burning.
My conclusion is I couldn't find sweet spot with adjustments (& lack of experience/knowledge) with last year’s pellets and just got lucky with this year’s pellets adjusting nothing. Stove also runs cleaner which is bit baffling since with these pellets I should be cleaning my Harman stove every 4 days versus the 7 I usually get by doing while I can extend the time between cleanings on my Bixby.
Good luck and interested if you find solution since I'm guessing will be in same position again depending on what I burn in future. And I feel for ya, I had mega time invested last year and my frustration level was high. Guess if there is any help for you in my reply in my case now see my problem as an adjustment issue not a stove problem issue.