Your high voltage could be caused by a bad regulator. Before replacing it, I would check all your grounds. Check voltage from battery neg post to the engine, frame, firewall and most important, the regulator itself. These readings should be around 0.2 V or less. If the regulator is mounted on three rubber feet, it must have a separate ground to its base.
Fla'da boy this could be the same problem you are having.
The internal reg on your SI-10 or SI-12 is held in place with three screws. Two of them are tied to the brushes for the rotor. The third screw is the ground connection for the reg. If this screw is loose or making a poor connection, the reg is floating electrically above ground and can not control the current to the rotor and therefore can not control the alt output.
As Wrenchbender said, check the voltages and grounds with the engine running.
