Is it possiable you are getting a partial siezure? A friend of mine had a similar problem on an RGV250 but he was doing 130mph (well topped out so what ever that is) and the back wheel locked for an instant but then every thing was fine again...it did this 3 times, and he found out that the plating had worn out beyond its limit.... but hey it could be a completely diffrent thing. worth a look,
I have a complete gear box from another bike in perfect condition, its a spare for my 21, if you want to send pics of your BIts (oh errr ) i can have a look to see if they look worn or broken compared to the box in my garage, Hard to do with pictures but if your really stuck might be worth a try..