When driving on the motorway i can get upto speed fine(70mph), but when i slow down and let the car coast(not apply the brake) the car seems to jolt(when changing down from 4th to 3rd). Once this happens, if i try and switch to manual mode this doesnt work(dashboard doesnt change). The car also seems to go into default mode and rev's more than usual. If i carry on i get 'Engine System Fault' message on the dashboard. I think it may be a gearbox issue but just thought i'd ask for advice incase anyone else has experienced this before.

Thanks