There must be a wiring problem somewhere.
Buy a new good quality voltage regulator such as this one: # C8313 - Heavy Duty External Chrysler type, Voltage Regulator with Adjustable Voltage
Mount it solid to ensure it's grounded well and/or add a grounding wire to it. Run 2 new field wires. The closer you mount it to the alternator the easier the wiring is for the field wires, generally speaking...