2000 Renault Espace gears

I have a renault espace automatic petrol year 2000.
Sometimes when I drive the car the gears don't seem to change and the engine revs are really high so that I have to slow down to about 30 miles an hour but if I stop and manually go through the gears 1 2 3 then back to drive, it usually drives ok again, but not always. Occasionally the electrical warning light comes on.
We have had renualt change the oil, had it in for a diagnostics test and they said it came up no faults but recommended an oil change which they did or that at 80,000 miles recommended a new transmission. We've cleaned pipes out that we could loosen off. We were told it was a vacuum transmission and if there is any holes this could be the problem tried to check all pipes and degunged them.
At other times the car drives perfect and reachs top speeds with no problems you never know when it is going to happen again. Any help, or advice grateful