RE: Let's talk thread lock (and shimming)
There are two ways to have an improperly shimmed clutchbell. Too loose or too tight with the former being more likely with all other variables being right.
When you say the bearings are "good", are you taking them out of the CB and checking for lateral play? If the ball bearings or the races that hold them are worn, the bearing will spin "like its on ice" but will be loose with side play.
I'm not picking on you - but if it is tight at the start and comes back loose, something isn't right.