I am no expert, so I've been reluctant to reply. But your problem is almost certainly a broken speedometer cable (or less likely, loosened nut at the back of the speedometer). Same thing happened to my '68 280SL, manual transmission, about one year ago. A new cable from PelicanParts for $36 fixed my problem. An added benefit was that my speedo had been "bouncy" at low speeds and that also went away (I had feared it was the dreaded slipping drive at the transmission). Replacing the cable is fairly easy.The worst part is accessing the back of the speedometer at the dashboard, but that has been throughly discussed in this forum. Good luck!
-Enrique Garcia