When mine did this, a simple exchange of the alternator took care of the problem. I also checked for loose wires and other shorts, but it was the alternator.
P.S. - Mine also flashedm, so I didn't immediately assume it was a bad alt.

-Austin


1988 SWB Montero - 2.6L - 5-speed
Weber Carb
(Coming Soon) Rugged Ridge AVM Hubs