Sounds like maybe the regulator is failing or stuck... Either way, I think you should replace it. But, before you do, check something. You say that you get a reading of 15. I am going to assume 15 volts, but where? At the battery? If so, yes it is OVER charging and needs fixed. The max I like to see on a 12V system is about 14.6. Different people would give a number close to that one. But either way I think we would all agree 15 is too much.
If, the reading you are getting is NOT at the battery, check the ignition switch and the fuse holder. I have seen a poor connection at either point cause the voltage regulator to "think" it was trying to charge a weak battery, when in fact it just wasn't getting signal to reduce voltage. With the motor running at about half throttle, reach in and wiggle the plug on the back of the key switch and see if the ammeter drops. Do the same at the fuse holder. If either feel hot to the touch, that is also a good indication of a poor connection.
Good luck!
