That can indicate a low battery just getting it's charge back up, but I find that when the gauge gets swingy, It is usually time to clean the ends of the battery cables. Don't forget the main cables have two ends eh, the other end is the one that bites you usually for a bad connection.
I also find if I polish up my fan belt the gauge can get swingy and kill the battery slowly. To test for that I take a cold off engine and see if I can hand slip the alternator pulley. If I can, it for sure won't hold under load and either needs tightening or replacing. A loose one usually makes noise, a polished one just slips. I killed 3+ belts a season when I used to mud run. Mud=liquid sandpaper.
86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10. Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com
(10 new albums added Sept 16/10)