I believe you have multiple problems. First, the voltage at the battery should never drop below 13.2 volts. Is the alternator the proper amperage for your car and accessories? Is the harness from the alternator to the starter solenoid in good shape? If you have broken wire strands in the harness or at the connections, you will get the proper voltage with no load but it will drop with a load on it because you have no current carrying capacity. Second, you may have a bad ammeter (I'm assuming this is the gauge you're concerned about) or a wire that is shorting. Ford ammeters are only marginally useful to start with. They sample the voltage drop across a short section of wire in the main harness, instead of running the voltage through the gage, and aren't really sensitive enough to accurately portray what is happening. If the ammeter is jumping around, look for bare wires, especially below the radiator or where the main harness goes through the core support. If everything looks ok, you may have a bad gauge, although long distance trouble shooting is a dicey thing.
Steve