Problem with lights
 

Hello, I have a 2005 GMC sierra crew cab Z71. I am having a problem with my driving lights and interior lights. My volt meter is above 14 while the truck is running and any light I have on my truck will fade from bright to dim and then stop and work fine and then do it again. They don't totally go dark, they just go bright and then dim in a rapid manner. It will do it for a little while and then stop and then do it for a min or two and then stop. Very weird ! Any help would be greatly appreciated. It's almost like on a police car when they turn their emergency lights on and their headlights go bright to dim in a rapid manner, it's almost like that except not that noticeable.