My 96 S320 Engine shakes uncontrollably at times. It usually starts when I am driving and it does not matter the speed. Once I pull over I can see the front end of the car shaking. But once I turn the car off and turn it back on it stops. What is this? I was thinking that I need new motor mounts but thought it could be something different, since it goes away once I restart the car. Can anyone help????