Hi Martin, I was looking over the whole project again and wasn't quite sure about something. Did you end up deciding to leave one brick removed as an ash and ember catcher? if so, is this essentially the first brick you would see looking above from the feeder side? My thought is that maybe you should try replacing the brick and insuring that your highest concentration of heat is not directly below the feed opening. Is it possible that you have a large accumulation of oxygenated fuel causing an updraft there?
Also, I like the suggestions you are getting for troubleshooting! It's nice to see so much support! Good luck again.
P.S. the good news is while you try all these things you are drying out your system, so it may start performing better anyway.