So... everything done on the car, full internal external overhaul of the gearbox and my mechanic took it for a test drive and after 20 mins aprox, when the ATF gets warm, it drops out of third gear. If he lifts his foot of the gas, it shifts again through all gears. When the ATF cold, all works as it should. It first engages 3rd gear than drops it, falls into neutral revving the engine. All pistons, valves etc were put in new or overhauled, vacuum and bowden put back and laser adjusted. What can cause this? 😭