Why would the voltage gauge on your 1997 Chevy blazer be fluctuating?

Your alternator may be on it's way out. Places like Autozone will put a machine on it and check it for you for free.........you coudl check to make sure the serpentine belt is tight on the alternator pulley. There's also a possibility the bearings in the alternator are getting dry and starting to get tight and will eventually seize up and then cut your belt when it does. It could be the gauge, but I would get a free check up at your local parts store first.