Have not seen it mentioned here yet, but perhaps your voltage regulator went out. I usually toss a new one in when I replace an alternator. You say the FLAPS checked the battery and confirmed it to be good, and that you replaced the alternator a coupla years ago. Might be the voltage regulator.
do you have a voltmeter? Like in your dash. I have a triple gauge that tells me oil pressure, coolant temp, and system voltage. if you are getting about 12 volts or less at cruising speed, say 1200 rpm or more, battery and alt are new, shiny connections and grounds etc, sounds like it might be a regulator. My 292 idled around 800-1000 and always had about 13v at that speed.
Nope, only idiot lights. Like I said, I just moved and am still trying to figure out where I put my multimeter. Someone walking by had suggested the regulator too.....but it's internal on the '78 alternator. I'm sure I could change it with the alternator apart to re-clock the position of the connectors so they are accessible in the stock location. I'm also still eyeballing the starter as a contributor to the issue....38 years is pretty old. The positive battery cable maybe isn't in the best shape either.
