Carl,
I have a G-Box 3000 and don't have many problems. You mention that the positioner is old. Do you know what type of sense mechanism it uses? My set up uses a reed switch (two wires), the box supplies a bias and the line is pulsed by a magnetic switch. The is another type of sensor, (optical of magnetic) where there is a wheel with slots in it. As the wheel turns it pulses the sensor, in this case you may have to supply a higher bias voltage or reverse polarity to get the sensor to work. IMO the reed switches are much more reliable....simplicity.
Aside from the sensor your motor may have (most do) a physical limit switch problem. There is a cam that engages a switch at either side of the arc. Something may be hung in there. I set the software limits just shy of these physical limit switches
You may also have the GBOX software limits set on top of each other, say west limit is 1000 count and east limit is 1001.
You can disconnect the sensor wires and try to move it with the GBOX, just be careful to watch the dish for movement binding. In my case my receiver was commanding go east while the dish was moving west. That messed with me until I got my butt off the couch and moved receiver, controller TV outside. I just reversed polarity on the motor control leads.
Hope this helps.