hello there
happy new year.
if i was you, i would first remove the left hand side engine alternator cover and check the gear selection lever mechenism for excessive play and wear. and that the gear box drum controll fork is well centered with respect to the drum needles. if this fails then removal of the gearbox fork drum and selector forks should be carried out for inspection of excessive wear and play.
i own a 1989 750 sport, whicvh i believe has a similar engine to the pantah.
i am no expert, but i hope this may be of some help.
you could try contacting jon palmer on 01594 836 067 who is an ami ducati certified mechanic.
cheers
shaun