Alot of bouncing ????? Help

[Sambecky1989]

Hi all

I have a

Vw passat b5.5 2001. 1.9tdi 130 saloon

Now I need some help

When I stop at traffic lights or just sat still with engine running the car is shaking like bobbing up and down ( that's what it does to you if sat inside the car while it does it)

It's only on idle
But doesn't do it constant just now and then ????
Is this normal ??? What could be causing it ????

Thanks