A6 3.0 Violently Shakes When I Start

[Speed_Speed]

Guys I have an A6 3.0 -2003. Once every 3 to 4 weeks when I start the engine and try to put it in reverse gear it shakes very violently. It jacks for a few seconds and I have to slam on my brakes to stop it from shaking. It can go for a month or more without this happening. Then when I least expect it, it will do that. This only happens when I start the car and try to reverse.

It has about 42K miles on it and I got it Pre-Owned. I have taken it to local dealerships and they keep saying there is nothing wrong with it. Because this happens every once in a while, there is no way the dealership can know when this happens.

Any help will do guys.
Thanks