My 1972 FJ 40 has a standard 40 amp alternator and separate firewall mounted regulator. When I start it up cold the volts are 13.5-14. However as I drive around it drops and after about 45 minutes of driving Im down to 11.5-12 volts. no mater how much I rev the engine it wont go up. If I shut it down for a while and restart its back up to 13 but soon drops to 11.5 to 12. The alternator and regulator are about 1 1/2 years old. I have checked my dash gauge with a separate voltmeter and they are the same readings. Any suggestions? Thanks