C5 Petrol Automatic Gearbox Problem?

Message displays "Gearbox Faulty" when started cold. All gear positions light up and gears won't change up. Have to stop the car, turn off engine and restart, then the problem goes away. Garage hooked it up to diagnostic machine, no fault showing, so they are suggesting sensors might be the problem. This would cost around 1000 euros and no guarantee it would solve the problem. Where can I get help. When guarantee runs out, nobody's wants to know, least of all Citroen. Anyone else have this issue?