Voltage drop on the highway...

[Capri306]

If you have the stock three-wire harness still on there, I would replace that while you're at it, if it were mine. I'm speaking of the connector with the larger, black wires with orange stripes. The connectors in those like to lose tension, and consequently create heat and start fires. New harnesses are available at most parts stores for this reason alone. I got sick of the low output of the stock alternators and converted it to an SN95 style. Never looked back.