df 140 gearshift problem

[nan-ber]

anyone have any gearshift problems with this particular motor, lost reverse the last day for no reason then it came back and for the next few days it would only work after it got a run and the gearbox was warm, i checked it all out and cant find anything really, control box and cables are all new this season and am convinced its in the box, which i stripped and found nothing worn or damaged, the only thing i found was the selector rod seemed tight but for no reason cos its all clean, anyone got any ideas . thanks