I have a 1993 Ford Crown Vic and my voltage meter on my dash keeps “dancing” while I am idling. It does it more if it is in gear but will also happen when not in gear. Alternator, battery, fuel pump, water pump have all been replaced. You can actually feel the engine pull down when this happens. Also, sometimes it actually causes the car to shut off but it will crank right back up.
Now, comes the harder stuff: checking the wires which carry power, starting from the battery and alternator. Check all ground with equal care. Power supply electrical schematics would ease the task. Oh, yeah, and a multimeter.