Loose wire, happenend to me in my probe when we did the clutch. Forgot to take the speedo wire off and snapped it. When we put it all back together we attached the wire best a we could but it came loose and made the speedo work sporatically at best. A lil WD-40 helped it. It souunds like yours is some kind of power wire with the same thing happening and since the cruise is controlled independantly of the speedo it is fine.
