Just speculating but sounds like a loose belt.

Rich Blake

Date:    Sat, 28 Sep 2002 00:15:18 +0100
From:    Anthony Polson <acpolson@HOTMAIL.COM>
Subject: Alternator warning light problem

Hi y'all,

Today I did a 423 mile round trip in my '84 VW Transporter-based Reimo
camper a.k.a. "Vera Vanagon".  Soon after starting the outward journey,
driving at 65 mph I noticed the alternator warning light flickering at me.
I slowed down, expecting to have to stop but the light went out at about 58
mph.