I had a similar problem on my 748, the lever kept locking up when I tried to change down a gear...
If you remove the LH cover (make a puller up yourself from a piece of flat stock steel and some bolts), then remove the flywheel. Behind the flywheel you will see part of the gear change mechanism which consists of a fork with a pin in the middle. The pin has a slot in the end and a locknut.
This pin has an eccentric adjustment on it and there is supposed to be an equal gap between each side of the pin and the fork. It has possible that the pin has rotated slightly causing a unequal gap (it had on mine...).
You will need to remove the mechanism to measure this gap properly, mark the position of EVERYTHING with an indelible pen (including the adjustable plate and the selector claw) this will give you a rough idea of where to start from when you put it back together. There is a line marked on the selector claw, this has to be halfway between the pins on the drum. Make sure you can change up and down smoothly before putting the cover back on.
Use a new alternator nut and torque it to the correct figure using a little bit of loctite.
Let me know if you want any more details.
Good luck!
