speedometer works strangely

[jimfulco]

My '79's speedo has lost its mind. It reads inconsistently after 10 to 15 minutes of highway driving (70-80mph). With the cruise control (aftermarket, uses driveshaft magnets) set at 75 or 80, the speedo will sometimes go down to 65 for a while, sometimes as high as 95. I finally caught the thing jumping 8mph or so upward yesterday when I hit a bump, & I can't figure out what would cause this. It doesn't wag back & forth like it has a dry cable, it just reads wrong. Any ideas?