control unit
I would bet that it would be your shift cable. Try as suggested to lube it between the cable and the jacket..(it will take a while). Also, if you noticed from before that the shifting was more difficult after the engine had been running for a while, I'd be even more sure that it's the cable...as it would expand with the heat, and given age and deteriorated lube, the friction with the jacket makes it nearly impossible to move sometimes, although it may move quit freely when cold....it happened to my throttle cable. I needed my full weight to move my throttle when hot, but cold she adjusted easily.