Time for my first big cleaning. Reaction chamber temperatures have been dropping quite a bit. I was getting 1400-1600F the first several weeks, then for a few weeks it was peaking around 1200F, and now it has been having trouble reaching even 800F. Cleaning the reaction chamber ash out helped the first time, it hasn't been helping since. I let the fire go out overnight and the heating oil kicked on to replace it. Yesterday I tried splitting a bunch of wood smaller than I had been using and that helped it to get up to 950F or so, but still not where it should be.
I figure I will start with the primary and secondary elbows, but I suspect I am going to need to remove the primary air channels and check those out too. Or I've just allowed too thick a coal/ash bed to build up after over-reacting the wrong direction to having it go out on me a couple times due to small loads in shoulder seasons. Heat exchanger tubes still look great, just a little fly ash in the chimney tee, but I am definitely getting a bit of crusty creosote buildup in the firebox near the primary outlets. Not enough to cover the holes, but I don't know what is inside those channels yet.
If any of you have seen this specific behavior before I'd be interested if you know what fixed it most directly.
Those wire harnesses attached to the stepper motors are locked in good, may try just cleaning it without disconnecting them (after cutting power at the breaker). Cold days coming up tomorrow.
EDITED TO ADD: I think it was just too much ash and coal piled up in the firebox. The weather went downhill faster than expected so I didn't have time to do all the maintenance I wanted, but the secondary charge tube looked fine and so did the elbows, I did not make it to the primary air channels. But I have it humming along above 1000F now on wood that is basically kindling, so it seems like the firebox cleaning was what I needed.