1996 Civic Jittery Speedometer

I know this is a common problem and I've scoured these newsgroups for
answers but have come up empty. Plenty of reading material but no
definitive clues as to the cause of the problem.

So, here goes:

Sometimes the speedo doesn't work at all.
Sometimes it dances around but is obviously not correct.
Sometimes it appears to be working fine.

Other pertinent info:
- The odomoeter is also affected. It's not racking up miles the way it
should.
- There are no engine warning lights lit and the car runs fine.

From what I've read, the speedo does not use a mechanical cable but
rather an electronic link to a vehicle speed sensor (VSS?).

My questions are:
- What is the most common cause of the jittery speedo? Bad VSS? Bad
connections? Bad speedo unit? Something else?
- Assuming an answer to the above, is the fix a DIY job?
- Are there legal issues to deal with if I have to replace the
speedo/odo unit? I have seen some used units for sale on e-bay. Where
I live (Ohio), I believe the vehicle title has a provision to indicate
a faulty odometer.

Thanks in advance to all who shed light on this.