White steak on Dell 3000cn, with new toners and drum

We have a Dell 3000cn printer that is printing a white streak down the middle of the pages, but only in the black ink. We have replaced the drum with a new one, put in a new black toner, tried the cleaning bar, removed each of the toners and reinstalled them to make sure they are in all the way and nothing has worked. Any more suggestions? Our IT guy is thinking it's a bad toner cartridge, but I want to make sure before we spend the money. Thanks!