There will often be some scooting up and down when you set bias via fuse holders - I usually let them stand for 10-15 min. However it sounds like yours is a bit more erratic - is that for both sides ? At least try both sides and if one side locks in bias smoothly you can swap driver boards and see if fault follows. On that note be A) careful removing and re-inserting the TR815's from their sleeves B) check TR815 that the 3 leads are making good connection, while the 2sc281 (TR815) transistor rarely makes trouble, its leads can dislodge if the boards have been moved around or worked on, this could certainly cause biasing issues as this transistor acts as a thermal feedback for the bias circuit.
As for alternative setting procedure, I just described that in post 20.