Transmission gears down on 1990 Mercedes-Benz 300CE

Why does my car gear down while driving down the road? It almost stops, then takes off again, doesn't know what gear it needs to be in.

Asked by for the 1990 Mercedes-Benz 300CE
There is a shift linkage on your car that may need adjustment for the transmission to kick down a gear properly. Mercedes call these bowden cables. There are also vacuum actuators that can fail. This will probably need some diagnosis to determine if this is being caused by the bowden cable, vacuum problems, or something else in the transmission.
Do the symptoms only happen when you're passing a repair shop?