You might need a full rebuild, but...
On the shifter thing inside the tranny, there is a grooved piece which engages the rotating parts and slides them around. This grooved piece wears. Easy and cheap fix.
Pull the tranny sidecover and inspect. It will be very obvious what the problem is, if indeed this is it. Remove the old one (circlip, I think) and get a new one.
Good luck!
George
