Well, since I didn't get any answers, I thought I'd post the solution:

At some point in time, someone greased the back of the speedo where the cable goes in. After time, the grease ran down the cable and into the sensor, screwing up the sensor. Had it cleaned and it works fine. I understand this isn't an uncommon problem.

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Bill Streep
San Antonio
'57 190SL (toy)
'08 S5500 (mine)
'09 CLK550 (wife's)
'06 SLK350 (daughter's)
'11 GLK350 (daughter's)
'03 CLK310 (spare)