I doubt it's your regulator, I had the same thing happen to my bike a few years ago and it turned out being the wire that feeds power from the the regulator to the battery in the wiring harness was undersized and it didn't provide a good charge... I bypassed the harness and ran a wire(12ga. if I remember correctly) directly from the regulator to the positive battery terminal. All it took was a splice and a connector for the battery. You can see in the pic, the black/red wire comes from where the rectifier hooks to the harness, spliced to the red wire which is direct to the battery.
