Bongo Fury

Help!!!!
96 Bongo auto gearbox has not been able to change up from 3rd to 4th gear, otherwise drives fine, however, yesterday whilst driving at about 45mph on the motorway it changed up to fourth, then couldn't decide which gear it wanted to be in changing between 3rd and 4th staying in fourth for a while before rapidly switching between the two gears whilst under the same load. After leaving the motorway and accelerating away from the roundabout it settled back to sticking in 3rd gear for the rest of the journey. Have previously replaced TPS and had serviced and gearbox oil level and condition checked by a specialist. Only other thing I have been doing is adding redex to fuel to clean engine to improve starting and cold idle, which has worked.
Any one experienced anything similar or have any suggestions as to what might be the cause?

Hello.
Not sure if it will cure your problem but it certainly won't hurt to not only change the transmission oil, but also drop the transmission sump and clean out the filter if not been done recently. I recently done mine and it's a relatively simple job. Do remember to have the engine running when checking the oil level.

if you have replaced the tps did you just put it in the same position as the old one or have you tried to adjust it, this controls when it changes gear with how much peddle.