My guess is the dampers are getting clogged and starving the head for ink.
As you print, oversized clumps of ink in the damper get shaken up and sucked into the screen. Over the years these oversized particles build up in the damper. When you stop printing and the ink flow stops, the particles fall off the screen. Also, the quicker you lay down the ink, the more you agitate the large particles which equals greater suction into the damper screen. High-speed banners will be the worst, you may be able to limp along on your highest quality settings and or slow down the head until you get new dampers...
If you check your nozzle dropout and then do a head cleaning and you lose more, It's probably a damper.