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.
1991 164B Silver Daily Driver
1991 164S Red rebuilt engine
1973 Berlina rebuilding a SPICA engine
1972 Berlina with transplanted 1984 engine. Daily Driver
1991 164S Black now stripped parts, parts and more parts