Jaguar S-Type automatic transmission problem

hi,when driving in town gearbox works fine.take it on motorway problem starts.when in 3rd gear at approx 60mph accelerating waiting for it to change to 4th it hesitates when it does change it goes in with a bang and the whole car lerches forward.sometimes GEARBOX FAULT comes on.switch car off and message goes off.it does not come on every time car hesitates to change to 4th gear.taken it to jag garage they could not find any fault codes when on diagnostics.dont know if this has anything to do with prob, but when car cold select reverse gear car lurches backward,but not when warm. many thanks.kennyboy