When the secondaries kick in its noticeable, no mistaking it. The only time I can drop my temps rapidly with fan is when the unit is starting up from cold or burning down. When stove is hot w/ secondaries going, I can often crank the fan and still have the temp go up. A drop in temp of a few degrees is understandable, a rapid drop from 600 to 300 w/ a fresh load of good wood is not my experience anyway.
Not my experience. Wish it was.
The wood has to be mentioned because it's such a common problem. The fact that your glass stays clean is some evidence that the wood is OK, but don't absolutely rule that out for the time being. You say you bought wood from a dealer recently and that its good? Where are you located?; cause let's just say, that's uncommon

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I am located in SE Mich. This wood dealer is a real character, as one can imagine. I know it is uncommon. Got to be seen. Plus the mountainous pile he has outside in back of the huge barn is staggering.
The fact that you had to add to the stack and that it's still just w/in spec w/ poor secondaries could point to draft. Have you dropped the baffles to clean it out and checked the flue and cap? I've had a fair amount of buildup on the cast iron baffles especially my first season.
Haven't done anything like that yet as far as baffles. The flue and cap are fine and brand new.
Try to post pics of the outside w/ roof line and so forth. It's above my pay grade but others here know that stuff good and what the impact on draft could be. I can say good draft is required for secondary air/burn.
Will do when I can. Not going to happen today. Sorry. Gotta head out soon for a few hours.
Puzzler, need some photos.