1986 300SDL – Speedometer Revving
 

Last night I was in the turning lane, waiting for the light to change, and I noticed that my speedometer was repeatedly "revving" from zero to forty. I was at a complete standstill and the engine tach was registering a rock-solid 6k rpm (where it's set). I pulled over and let the car "take a break", and it hasn't done anything like it since, but it weirds me out.

Time for a new instrument cluster or something more sinister at play?

You mean 600 RPM, not 6000 RPM, I assume. I'd hate to have one of those idling 6000 next to me (can it even safely reach 6000?) :).

yes, 600rpm. my bad.

well, the speedometer is electronic, I'd check the grounds to the body, check the ground contacts on the cluster, and check the speedo transmitter on the transmission.