Alternator

Driving on the highway for about fifteen to twenty minutes, with the A/C on my charging gauge goes down to about 12. Fifteen minutes later it goes back up to 14.6. Up and down for ten hours of driving. No problem starting back up and the gauge where it is suppose to be. Without the A/C on, it does not do this. People told me a bad regulator and some said a belt slipping. Three people have looked at the belt, it looks good. Battery and alternator are original.