Volvo: 240 erratic speedometer, added resistance, mechanical cable

Question
I have another problem:  my speedometer is nuts.  My odometer wasn't working, so I replaced a broken gear.  Now that works fine.  Before replacing that gear, my speedometer worked perfectly.  Maybe it's related?  A few weeks after I replaced the gear, I was on the highway, and looked at the speedomenter.  It was completely dead.  I stopped at a gas station and took apart my instrument panel, thinking a plug had come loose from my previous tinkerings.  Everything seemed fine.  I gave up and got back on the road.  Ever since then, my speedometer has worked- sort of.  It reads a steady speed (which seems to be off by about 5mph) once I get to between 40 and 60mph when I'm just starting off from park.  After that, it works until I slow down to around 25, where it suddenly starts waving around all over the place and then drops immediately to zero at any speed less than 25 (i'm guessing).  I've never heard of this problem before.  Any thoughts?

Answer
I have always had a similar problem with my speedometer, and I learned at one point that the mechanical cable linkage inside the cable body gets gunked up with dirt over the years, adding resistance to the spinning of the interior line (works kind of like a bike brake cable, except turns instead of pulls).  The transmission drives the rotation, but the added resistance allows a small amount of torque pressure to build up in the line before it finally does the corresponding rotation at the other end (i.e., your speedometer).  This causes the jumping of the needle as the end of the link  breaks the hold of the dirt in the line and spins out the build rotations.  One the pressure is released, the speedo end stops spinning (your needle suddenly drops) until the pressure is released again (your needly jump up sharply).  Now, I'm not sure how this problem could have occured based only on what you said you did, but there are several possibilities.  Your movment of the speedo line disturbed enough built up dirt in the housing to cause the problem.  You might have also damaged the spinner inside the speedo itself, causing is to catch in some way.  A good way to test would be to but a cheap speedo at a junkyard and see if it jumps in the same way.  If it does, then I would look to your speedo line.  You could either replace it with a new one, or try shooting oil or WD40 down one end in an attempt to loosen up the spin.  Hope that helps!