Sluggish Speedo

My '84 400i has recently developed a sluggish speedometer. It either moves very slowly or not at all. It seems to like to get stuck at about 40 or 62 mph. Other times it seems to work find. Even at rest is sometimes still says 40 or 60. The odometer works fine even when speedometer is acting up. I'm sure this is not a first! What is the problem and what is the fix? Is the speedometer mechanical or is signal electronic? Will a little dab of graphite in the right place fix it? I hate to think where what is will take to get to that right place!! I can drive with the tach, but it will be nice to get the speedo working again.
Ken

 

Dave and others
Where does the signal come from? Most cars with mechanical speedo's come off the transmission tail shaft. Motor cycles have to come off the front wheel because the rear is usually spinning, then again the front wheel can be off the ground. Is there a sensor on the front of rear wheels? Remember, when the speedo is stuck the odometer is still working!
Ken

 

Guys
It looks like my problem is internal to speed. I will add this to my winter list. PA Speedo is likely to get both my speedo and clock for repair. They are highly recommended by all.
Ken