Sounds a lot like the problem I had with my TR6C. Third gear was not staying engaged. Solved it by shimming the mainshaft. Here is the thread:

http://www.britbike.com/forums/ubbt...;Main=43805&Number=396860#Post396860


47 Norton ES2
61 Norton Dominator 88
67 Triumph TR6C
68 Moto Guzzi V700
74 Norton Commando
08 Moto Guzzi 1200 Sport