If the horizontal lines are lighter than what you are printing, then you are not dropping enough ink - either your resolution is too low or your nozzles are clogged. If the horizontal lines are dark, you might have a problem with the belt or the encoder wheel since the printer is not stepping the platen the proper distance and the print head is printing the small portion twice (i.e. giving you a dark line).
Sounds like by what you put above, you either need to run a nozzle check to see if your white ink nozzles are firing properly. Run the nozzle check on a piece of film positive or transparency so you can see the white ink channels. With you having to do an ink charge to get it looking better, it sounds like you have an ink starvation problem. Not sure if you are monitoring your temperature and humidity, but that might also have something to do with it.
here is examples of lines appearing, its more visible in under base and printing colour on white tees.
Check your pattern check. This looks like blocked nozzels on one or more channels. settled "TO" could cause this, or if you're not keeping the head and capping area clean. Also, if this happens durring a run after having a good nozzle check, 2 suspects would be 1) humidity too low. 2) ink running low in one channel causing starvation.
Faint lines in print 
