I would suspect an alternator is going out. That would be my best guess since a battery is not monitored. Get your alternator tested.
2004 GMC Sierra Extended Cab
-K&N Series 77 CAI
-TB Bypass & removed insulators
-Catback System with Stainless Steel Exhaust Tips done by Mr. D's Muffler shop off East Hardy & Gulfbank
-Factory 20s with Michelin LTXs
-Spray n liner with a Truxedo bed cover