Truck suddenly stops moving while driving

After driving continuously for one to two hours, it suddenly disengages with gears system (go neutral ), after switching ignition key off for a couple of minutes and start engine again driving becomes normal. Gear box was recently maintained ( replaced about fifteen pieces of gears) because of repeated "clicks" in gearbox. Mechanic replaced what he called " air sensor ", but the problem of the "sudden " of gearbox disengagement still on. Please advise.