I've had the same symptoms a 1970s UK car (Lucas Wiring) when the instrument voltage stabilizer failed.
This was a small metal unit that looked just like a flasher unit (and worked on the same bimetal strip principle) See the picture.
I don't have a Pantera wiring diagram to hand - but I assume it must have one of these somewhere in the instrument wiring.
Detom, does the temperature appear to drop if the engine speed drops below the point where the alternator stops charging?
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