Have had the same problem, my local Alfa dealer, Bishops, tell me it is because the wiring loom has been manufactured too short. When the engine vibrates over time it pulls the connection off. They repair it by putting a spacer in the connections however this is only a temporary cure, mine has failed three times now. The only real cure is to fit a new wiring loom, but that is not covered by the dealer 2nd/3rd year warrenty & would cost £800+
Apparently the fault is fairly common on the new JTS, but almost unknown on my 2.0 TS( no consolation to me !!!), obviously I have no idea if it has happened on a diesel, but it does sound a bit familiar.