Do you have an aftermarket stereo in there? GMs in general are known to come stock with just enough alternator to push the stock electronics. Someone a while back actually quoted or posted a link to the GM Bulletin telling dealers that voltmeter fluctuations are to be expected with aftermarket stereos.
Also, how old is your battery? If it'* getting weak, it'll have "dibs" on the juice your alt is putting out, thereby starving the rest of the systems.
Mine fluctuates a bit as well, by the way. It used to be really bad, but then I got a new battery...
1998 SE Midnight Blue Pearl
Gutted airbox, FRAM AirHog, removed MAF Screen, eBay clear corners, extended brake lights
I'm so much cooler online