1985 Renault Trafic Auto T1000 gearbox problem?

A friend has asked me to look at the vehicle in question. It had been stood for 2 years but was happy to start and run then pass the MOT fine. Then a trip to Tan Hill and up all them hills seems to have caused a problem with the Auto Box. It judders badly trying to change gear but all gears are there. I have told her to get the ATF out and fresh into it before we start thinking Gearboxes out. Any ideas as to what may be wrong please let me know.