I have had this happen on nearly every MB I own, and the first time I figured it was possibly already broken, so I would just agitate it some and see if it was really broken. If not, it might start working, or, it might really break. Or it was already broken. Anyway, when they jump a little while you drive, I do not think they are broken. I have always been able to punch the reset knob/button a few times and get them unstuck. I have even resorted to doing this when the car is moving if it did not work when it was sitting still.

I think the source of the problem is not pushing the knob or button all the way in when resetting it. The unit is not fully reset and cannot engage the drive gear as intended.

Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)