I have a very similar problem with my car aswell. For sometime the idle would get so low the car would even stall after shaking and in neutral it seems to go back to normal. So far to sure this problem i've swapped my OVP, and this prevented the car from stalling but the low idle and shake continues. I've done my engine mounts aswell, but the problem remains. On my car, when the idle gets really low for example in a Drive Through line up, I use to hear an exhust rattle that i've nailed down to my cat.converter. I don't know if my cat needs to be changed to stop this problem or if its something else. Does anyone thing it could be the timing chain tensioner?

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My Cars
Mercedes CLS
Mercedes C-Class W203 - All time favorite

R.I.P.
Mercedes C-Class - W202 Lemon
Mercedes ML - Warrior
VW Jetta – Worst Car I ever owned - Alot of problems