My thoughts may not have been clear.
The VSS at your transmission is sending a signal to your cruise control box. You said when operating the cruise control that the speed was fluctuating (not steady), unless I misunderstood.
By removing the VSS, you can see whether your speedometer is not steady, which would indicate either a defect in cable inside the casing or a bad speedometer drive/driven gear. If the speedometer behaves smoothly with the VSS removed, the VSS may be providing an incorrect signal to your cruise control box. I suspect that with your new cable, drive gear, and driven gear, that the speedometer operates smoothly. The next likely culprit could be the VSS providing a bad signal to your cruise control box, wrong switch settings, or a problem in the wiring between the VSS and the cruise control box.