If you've been running the same stove for 20 years and it is properly maintained, then I would look at the coal you are burning. Even if you get coal from the same dealer, you don't know if the dealer is getting it from the same source/mine. Furthermore, as our Leader has stated several times, there can be significant variation in coal from the same mine.
See if you can pick up a few bags of coal of known, predictable quality and give it a try. It could be that this year's batch of coal is simply not as good as the coal in prior years.