Since you are seeing the problem when hitting a bump, it just has to be a bad connection or shorting wire somewhere. Unlike other make cars, I've never seen a chafed wire on an MB. They do a good job running wires through bulkheads and such. That does not mean that there could not be a wiring problem in the middle of a bundle, but it is highly unlikely from my limited experience.

This leaves connections as sixto pointed out. Check where the wire is crimped on lugs. If you can have somebody watch the instrument while you wiggle connections or tap on the relays, you might find the problem.

This might be a really tough one to track down.

Best of luck,
Larry