Might want to change the caps anyway. We where having the same problem, we changed the caps and it prints much better now. I thought ours looked fine to but looks are deceiving. I look at it like this, it's a cheeper fix then the print heads. It's like $400 or so.
if the caps look ok, then it's probably the small rubber grommets that the caps move on. after a while, ink gets into these and the cap can't float correctly, therefore can't seal correctly, hence banding may occur.
you are alright for the start of the job, but as air has got into the system, it takes a short while to manifest itself as this fault. do a clean and prints ok, then banding. do a clean, then prints ok, then banding blah blah blah..
you can take the cap off and remove the grommets to soak them in the cleaning fluid to get rid of all the dried ink on it. also check the stalks that they mount onto. they should also be clean..
