2001 Sprinter 313 6sp ASG Auto sometimes loses auto mode?

Hi, we are touring Scotland in a Hymer based on the Sprinter 313 4 cyl turbo diesel with the 6 speed auto gearbox, two days ago as we were driving on a single track road with passing places - always stopping and starting,the gearbox stopped changing gear automatically but l could do it manually, as we never went above 3rd gear it was not a real problem but once on the main roads again found it would not go into a higher gear than 3rd, stopping turning off engine with the handbrake on and it all is back to normal for a while until we come to a roundabout or traffic lights when it reverts back to manual only and not higher than 3rd gear again but switching the engine off again then on resets it all back to normal for a bit ! can anyone help? Regards, John R