I would tend to believe that, since the note misfiring tends to occur
in the middle of the stack, something must be stopped up, worn, or
leaky.  I have noticed that both piano actions and player actions, if
showing signs of wear, tend to have more severe problems in the middle
of the scale, where the notes are played more frequently.

I have rebuilt many old stacks and never had this problem, unless I
chose the wrong materials or used the wrong procedure.  Even then, the
mistake was noticeable throughout the whole stack.

Check the piano action first, then the tracker bar tubing.  Was the
tubing all from the same batch?  Some tracker bar tubing, especially
from Player Piano Company, is very sticky and is a dust magnet.  Such
tubing will clog very quickly, and even stop up the bleeds in severe
cases.

Some modern XP QRS rolls have a wax-coated back.  If the tracker bar is
worn or misshapen, the sharp-edged holes in the bar will scrape bits of
wax off the back of the roll, and suck them right into the player ac-
tion.  I had this happen once on an old Baldwin Monarch.  Keep in mind
that, since the middle notes have the shortest tubing, the middle notes
are much more susceptible to debris getting through to the pouches and
bleeds.

But considering the fact that most 88-note players have the same size
bleeds, I do not think that the tubing is the problem, unless it is
stopped up.  I strongly suspect clogged bleeds, leaky pouches or wells,
valves, or a combination of these, or perhaps a faulty signal junction
or gasket.

Andy Taylor