Looking at the wiring diagram, it appears the speedo, cruise control, ACC, radio, fuel pump relay, CIS controller and idle controller all take a speedo input. It would be strange if a problem downstream of the speedo gauge itself would have no other manifestation. At the very least if the cruise control works when the speedo is flaky then the problem is in the speedo itself.

I didn't find any information of what goes on between the tranny and gauge. I know there is an inductive cable that looks like a speedo cable with no gear at the tranny end, and a device behind the speedo gauge that accepts the coaxial conenctor at the other end of the cable.

Sixto
95 S420
87 300SDL