Not sure whether this impacted the Plus Four but the early Plus Six cars had earthing issues, which caused random failures. In my case (MY23) the engine would randomly cut out and then run on four or five cylinders for a while. As both cars share the same chassis, I can envisage that this should be one thing to rule out, hence MMC asking the dealer to clean up all contacts and connections. One earthing point is behind the dashboard, I believe; did the dealer check that?