The infamous idle surge- a new clue?

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I've been fiddling around with my '93 300TE this afternoon trying to get to the bottom of the idle surge problem. I've searched and read through many posts and I haven't tried everything yet but I did find that my spark plug wire harness is in perfect shape.

I haven't had any problems with cruise control so I'm hoping that means the OVP is good.

With or without the MAFS connected makes no difference at all.

But I did notice one thing that I did not see in any other post... the idle ONLY surges in park or neutral. Once the car is in drive the surge disappears with or without my foot on the brake.

There is a very very slight hesitation off the line but there is no lack of power or unusual performance with my foot on the floor.

I've checked for vacuum leaks and found nothing but I'm not very confident in my check so that could still be an issue.

So shifting the car into drive is the only clue I have, does anyone have any idea why the surge would be gone once the car is in drive?