What would cause the following symptoms in an M46?

Sometimes, the amount of effort that is required to select a gear is so great that I have to grab the stick with both hands and "slam" it into gear. This could be any gear-the problem is not isolated to 1st or reverse, which rules out the reverse detent plate in my mind.

Sometimes, it shifts like new. It might shift perfectly for days at a time, but eventually the gears become almost impossible to select. Letting the car sit overnight seems to help.

I've changed the transmission fluid. Didn't help.

Could this be a failing release bearing? Check the cable tension and it can't be adjusted anymore. This would seem to indicate that the clutch is worn, but it doesn't slip or exhibit other typical symptoms of a 'bad' clutch.

Could it be an internal tranny problem?