skoda octavia 1.9tdi

Hi car will drive and change gears smoothly until i go over 35mph,then gearbox will go into low gears and gearbox gear selector lights on dash flash.I can pull over turn ignition off then restart engine and will be okay again but engine management light stays on,i got a friend to connect to his laptop and it says a unexplained fault code he reset it but same thing happened again.any suggestions welcome as main dealers are 120 miles away.