This sounds exactly like what my roommate's 1965 W111 220S does when slowing to a stop. It was converted from Zenith to Weber carburetors, and the Weber carburetors don't have a space for a throttle position sensor, which unlocks the transmission so it doesn't clunk when slowing to a stop.

All the automatic cars that were converted to Webers have this problem. In your case, it may just be that the throttle position sensor went bad. It attaches to one carburetor by the linkage and the wires go down to the transmission.

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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles