GdlmngEngnr

My Ford Galaxy MkII, 2002 automatic (102,000 miles) has a gearbox problem. Normally it's fine but occasionally (every 50 miles, say) the engine will rev freely as I drive along. Like it's been put into neutral, but I didn't do anything except accelerate or drive up a hill. It can be fixed by letting the car slow down to about 30 and putting the gearbox into top gear using the manual positions on the gear stick. Then it's back to normal. We had the garage partly change the transmission oil 6 months ago to try to fix this. Apparently the oil was in a poor condition. When I got it back it might have helped a bit with the gearbox problem but it's hard to be certain. Any advice welcome. e.g. Would a full transmission oil change help? I've been quoted £200 for that which I could live with if it fixed the problem.