1987 Jaguar XJ6 dashboard amp guage

Electrical problem
1987 Jaguar XJ6 6 cyl Two Wheel Drive Automatic 145ooo miles

The Amp guage shows below 10v when starting, warm up and after 1 hr of running on highway. I tightened squeaky fan belt, no change. I tested the battery terminals with ohm meter and have 14 volts. Head lights work on dim & bright, all electrical things work i.E. Turn signals, brake lights, wipers, radio, heater, pwr windows, interior lights, dash lights. My wife drove the car about 1 1/2 hr after she noticed the amp guage was below 10v. It starts with plenty of power. Idles like a kitten. I checked all the dashboard fuses hopeing for something, no luck.

Tom