Ah, electrical gremlins, and long distance too. Some questions first:
1. When you went to the show and had the tach problem, what was the ammeter indicating?
2. On the way home from the show, what was the ammeter indicating? +/-/0/slight charge/discharge???
3. Do you have an aftermarket ignition system? Does it have a tach adapter?
Assuming (fatal error) that the alternator, voltage regulator, and associated wiring are not at fault, then the only way for the ammeter to show a heavy charge is for the wiring on the battery side of the ammeter (the ammeter included) to have a wiring problem. This could be a grounding (or intermittent grounding) condition or a loose connection. This would include the wire from the ammeter to the battery, the wire (battery cable) from the battery to the starter solenoid, the yellow (I think) wire that goes from the battery terminal side of the starter solenoid to the A+ terminal of the voltage regulator, and the battery ground cable. I would check the battery ground cable and its' connection to the car body first.
Also, you could have gotten a bad regulator.
If you could temporarily connect a volt meter to the battery and drive the car (with the voltmeter where you can see it while driving), this would be the best way to see what's really going on voltage wise.
Below is a typical Ford charging system diagram. This is from the Ford book. Some of the wiring colors are different on the Pantera, but it gives you a good overview.
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