E220 CDI 2004 - Central locking juddering

[pcman_jh]

Can't think of another word to describe it but once every 4 attempts (sooner when it's colder) the central locking door buttons all jump up and down in quick succession for about half a second. They don't all stop in the open position, but sometimes get stuck in the middle position, so I have to press the remote again in the hope they 'judder' to the open position. They're fine when closing the car (although I think there's still the juddering happening)

Any clues - vacuum fault or a sticky electric contact that makes the 'system' think it's not opening.

Appears to be better in the warmer weather.

 

[television]

No vacuum locking on the 211, all electric motors, if they are all jumping is there an arcing fuse or holder, the bounce when a spring has gone in the locking motor, the other daft fault is that the locking rods pass through a rubber grommet id the door panel, the rods stick and the buttons bounce, spraying the rods where they pass through the grommets with silicon cures that