High volts

Hey guys, sorry if I hit a subject that's been answered a bunch, but IPhone search gets old fast. My charging system is pushing over 15V at the battery, regulator has been checked and changed three times, alternator is obviously charging, but way too high. I checked the field wire for shorts to ground-all checked good. It used to only charge 12.8V which made for interesting driving on cold wet nights leaving work, but now it cooks batteries in about 3 weeks or so. I'm struggling to figure out what's wrong with it...any ideas??

I had thoughts about either replacing the alternator, putting a resistor inline to reduce the volts going in, or trying to upgrade to a 1 or 2 wire internal regulated version, but I'm open to ideas...