What is the problem when gauges are erratic up and down? on 2004 Chevrolet Trailblazer

I got a new battery and checked alternator

Asked by for the 2004 Chevrolet Trailblazer
maybe there is an electrical issue
1 more answer
Ihad an alternator installed but now the check gauge come on and and the battery gauges drop all the way down whwat you yhink the problem is?my truck is a 2002 dodge dakota 3.9 engine