Sprintshift on a 2002 Rapido Motorhome?

Sprintshift works fine but after about 20 miles if I go over 55mph it will shift into manual mode. I can change up and down but it will not go back into auto. If I pull over and stop, switch of and then restart the engine it all works fine. If I stay at 50mph it is OK, but push it upto 55 and over & it will go into manual. I have had the oil changed and the vehicle mileage is 50,000