When I first start my truck my volt gauge takes 30 seconds before it even moves, then when it does it goes up to 16 then drops to around 12 and keeps doing this for a minute or two then it goes to around 15 and stays there like it is supposed to. It doesn't matter if my headlights are on or not but if they are they get bright and dim as it fluctuates up and down. Do any of you guys have any idea what could cause this? Thanks in advance.