It is my impression that all the instument needles in that IC (inst cluster) are stepper motors responding to a calculated value. That would make me think that the problem is more likely to be the motor than the signal (or the processor). Although poor circuitry could cause the processor to have bad output.
Does the speedo act similarly if accelerating with the "Accel" funtion of the cruise control. If the acceleration is smooth on the cruise control I would presume that the speed signal is good; which leaves the IC. Is it out of warrantee??
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician