Might be the first sign of a speedometer sending unit going bad. I had one do that several years ago. It got so I couldn't use the cruise control at all, then the speedometer stopped working. When I replaced the sending unit everything started orking again.
Red 1991 164S, Black 1991 164S, Red 1987 Milano, 1972 Berlina, 1973 Berlina rebuilding SPICA engine