I would look into increasing the mechanical idle speed..my car used to drop rpms below 500 briefly when it was cold and I was coming to a stop, most notably if I let go of the pedal rather quickly. I increased the mechanical idle (the small, 6mm screw on the back of the IP, above the ELR connection) to about 550-600ish, and this solved the problem. Well, it does still kind of drop down when cold, but at that rpm it does not have any effect on idle quality- no shakes, no stumbles, etc. Besides, the ELR setting of 4 should have you in the 680 rpm range, and that's with a ~80 rpm boost from the electronics.

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Dan
2005 E320 CDI - 246k
1987 300SDL TD05-16g, Herlevi pump, Elbe manifold, 2.47 LSD - 213k
Past: 1987 300D - 264k