Inconsistent redstone (dropper?) behavior

I'm observing some inconsistent, random behavior with a simple redstone contraption.

An "item elevator" or "dropper-vator" is designed to move items upwards and into a chest. There are three droppers, stacked vertically and facing upwards, with the bottommost dropper giving input to a comparator. When items are put into this bottommost dropper, the comparator output activates a clock (a comparator feedback loop) that outputs a blinking signal leading into the middle dropper. Weakly powered droppers also activate adjacent droppers, so this setup should be sufficient for transporting items up and through these droppers.

The problem is: Sometimes a single item gets "left behind" in either the middle or the topmost dropper. For example, put 8 (stackable) items into the bottom dropper. The clock turns on and you may hear a single "click" sound from the droppers (doesn't always happen - another inconsistency). When the clock finishes, you check the chest and find only 7 items. The eighth item was left behind in the middle dropper.

Worse still, this behavior changes, seemingly randomly, whilst playing (i.e. without quitting or pausing the game, without unloading the chunks that the contraption is in, etc). If you make some changes to the wiring to test something out, then change it back to how it was, you may find that the last item now gets left behind in the top dropper!

World save attached, of course.