Using a voltmeter, measure between the positive on the battery, and the blue field wire at the alternator. This should be close to 0v. If it's higher, something between the two has higher resistance and is causing the voltage drop, so the alt/regulator are regulating them selves to the proper voltage, causing over-voltage at the battery.
Good suggestion but I would check the voltage on the blue wire at the regulator as that is the reference voltage the regulator is using. Theoretically the voltage at the alternator would be the same as that at the regulator (still talking blue wire) but there may be a harness problem. The statement that when the engine is started the voltage is fine but after a few blips (and the resulting engine movement) the voltage is high leads me to suspect a bad connection. Shoot, it could be a problem with the plug on the regulator. Just have to cover all the bases.
