I haven't seen a regulator fail gradually or intermittently - only ever heard of them going all at once, and never coming back.
Have you checked your master (dogbone) fuse? It can crack and fail intermittently, but look absolutely fine on a visual inspection.
Also check the sense line on the regulator - the one that "turns it on". I don't have it in front of me right this moment, but there are the three yellow wires, the red wires that are the output, ground, and the sense, which when supplied with +12V enables the regulator. If that sense wire has a bad connection or is otherwise not receiving +12V, your bike will stop charging, and it will drain the battery.