I dont think your box has gone mate try the gear linkinge rods and that mine was doin same thing found out there was a pin loose that holds the rods 2 gether so i put a nut an bolt throught it bin sound ever since but double up the nuts mate on the bolt should be sound then ow them pins am a right prat to remove sum wd40 should loosen it hope this helps
well its not playing up now!! i did a autotest last sunday and it was fine. but want to get a f16cr to put in for a trackday coming soon! as at castle combe i dont use 5th so hopping a f16cr will do the trick.