First you must actually determine IF THIS IS the VR at fault 1....voltage drop in the ignition harness. This can be intermittent due to harness connectors that wiggle, vibrate, etc 2....voltage drop in the ground. IF the VR does not "see" the same exact potential (voltage) as the battery neg post, then you will have higher voltage. 3....bad brush holders or a bad rotor in the alternator. If one brush holder is broken, and intermittently grounding, on the "green" connection, it will "full field" and charge full output 4....if the VR are actually failing, the rotor in the alternator may be drawing excessive current. Read the shop manual. There's a simple procedure in there to check rotor draw. What all have you checked / replaced/