HI all

When i am on the motorway and entering speed restrictions for roadworks i put on cruise control for 50 mph. now 8 times out of ten i get a message Drivetrain malfunctioncontinue the journey at moderate speed full performance not available. it's ok if i turn engine off/on its been in to bmw who changed a sensor 1st time but it doesn't record a fault on the systems i dont want to take it in as they will charge for diagnostics if its not showing a fault. but its ok if i'm doing 75 anybody have a suggestion please