That could be a low voltage problem in one of the circuits. The problem could be caused by a loose or corroded connection. Sometimes a wire can go bad on the inside where the problem can't be seen but it can be measured with a multimeter.
Most auto technicians have a hard time finding electrical problems.
Two other things that can cause the speedometer problem are the speed sensor or the speed sensor gear. Sometimes the shaft and bushing are offset so that the parts can be rotated without having to replace the worn bushing.
