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.

Chris M
January 2010