Hello Snaak,
You may want to ask this question on the new SmokerBuilderU.com forum, that is where almost everyone from here has moved to.
Frank’s plan is to shut down the posting option on this forum once everyone has moved to the new site.
As for your question, Frank just addressed this issue a couple days ago on the new site.
From what Frank said, it comes down to not enough air flow. As you already mentioned, I would leave the FB door open just enough to keep your fire burning clean (no white smoke), hopefully that will take care of the problem.
Great looking RF by the way!