Speedo Cable

[Brent Kasl <kasl100@navix.net>]

What causes the speedometer dial to flutter back and forth by 10mph
either way while driving? I thought I had a cable problem so I
disconnected the cable from the rear tailshaft and connected it to a
power drill. I tried different speeds in reverse and the speedo reading
was accurate and held steady at all speeds. I checked my cable route and
did not notice any severe turns or loops. This cable is almost 40 years
old but thought the drill test would tell its condition. Any ideas?
Brent