Car battery gauge jumping?

Occasionally, my battery gauge in my car while im driving will suddenly drop and either stay dropped for a couple seconds or will return to normal. The gauge isn't bad because the lights will dim when it drops. I do have sub woofers but it will still happen with them off. I know its not the alternator because I just had it rebuilt and not the battery because I tried another one in it and the same result occurred. The only other thing I could think of is a bad wire but I would like to know other possibilities