Check alternator connections for being loose or burnt. Checks grounds and cables for corrosion. My alternator started growling at about 190 K but no surging up or down, I replaced it because it was annoying to listen to. You may want to check your charging out with a meter with all accessories on to eliminate possible volt gauge malfunction, does lights go bright or dim at night when it is doing it ? Also how old is battery ? Batteries getting ready to puke can make the alternator surge up and down, as well as take out a alternator in the process.