About a mile or so. ( a London mile or so). And that is with or without a trailer behind . Local gearbox boys are saying it could be a soloniod in the box? Today i Will be trying to get it plugged in to a computer to check for misfires and such.

W plate disco td5 auto. Got an intermittent problem , mostly when cold. Get up to 20 miles an hour then no gear change up and the truck over revving. Had new gear box oil and filter any ideas?