Congratulations, Jim!
For making it to work...
I cannot answer 2nd question, because I don't have
Mercury, but maybe signal is not strong enough? Do you receive other channels from AMC3, like MontanaPBS?
To check how motor works, best way to do is come close to it and use a short cable directly connected to receiver. If you see it working great with a short cable, maybe, you have bad connectors or bad cable some where in your house. Also, note, that when you move the dish to a channel on a Horizontal tp, receiver supplies more voltage and in some cases, motor even moves faster! Not sure, does the motor have "E/W" buttons to move it manually? If it does not move properly with input from these buttons, this means, something is wrong with it.