W123, USED Diesel Engine

I have just purchased a 300td. All is good except the speedometer makes a ticking/scraping noise until I reach speeds upto 60+. The needle doesn't move in a smooth motion but more like the second hand of a battery operated wall clock. Also the speedometer is off 10mph or so. Do I need to replace the cable or the whole unit?
Thanks.

I recently had my shop replace the ignition switch. Now the heater does not go off after the key is removed. I can't recall if this was the how it worked before, but it doesn't seem right. Before I return to the shop, I'd like to confirm this. Also, what can they possibly have done to make this happen? My understanding is that the wiring just plugs into the back of the switch. There doesn't seem much room for error.