Occasionally stops charging at run speeds

Low charge on voltmeter at idle then as engine revs. Goes to normal. Occasionally stops charging at run speeds. Has shut off due to low voltage.

At idle fan ac wont work and receive warning lights for battery, fuel gauge and volt gauge on dash shows below 12 volt charge. Usually corrects at run speeds, warning lights go out fan begins to work. Once, at freeway speeds, 65 to 75 mph volt dropped and car shut down due to low voltage.

Sat for 15 minutes, restarted car, volt gauge went back over 12 volt and car ran fine.