Haywire Speedometer

[pgls651]

Hi Folks
On a Pete 579:
Recently I had the speedometer completely turn loose and erratically going from near zero to about the speed I was actually traveling (GPS indicated). The very first signs showed a slight movement away from the actual speed but got worse with ascents/descents and rough roads. At idle, zero speed there was no dial movement- stays at zero. And of course the cruise control cannot work. The speed sensor has been changed, and this seemed to work for a few hours or so until the problem came back with a vengeance. Any ideas of where to look next?