Alternator gauge going up and down but battery and Alternator are both brand new

I have a 2009 GMC Envoy/Denali- V-8, and I am on the THIRD alternator-
two rebuilts and a brand new one since February of this year, plus a brand
new battery.  Here is what is going on.  I thought the two rebuilt alternators
were defective, so, they were replaced.  When the second one allegedly
failed, I told AutoZone I wanted a brand new one.  I had it installed last
week and, it was fine for a few days and now, after I drove about 60 miles
straight, the needle dropped from just above the halfway mark, to between
zero and half way, yet, the battery is brand new.  The connections are fine.  
Does anyone know what could be causing this to happen?  On the second
rebuilt alternator, the 5 month old battery was damaged, so, I had to
replace that, too, last weekend.   The GMC dealer says I need to buy only
a GMC Alternator, which is garbage, because they want to charge me over
$500 just to have one installed.  Something is causing the alternator to
charge errantically, causing the needle to vary, and I don't want to be
stranded.....