Sostenuto headache

[Clyde Hollinger]

I hope someone can help me with this.

Steinway B made in 1979.  Whenever the sostenuto pedal is depressed, I
hear a thump, and I will see several dampers jump, most notably damper
A#3.  After much hairpulling and trying moving the sostenuto rail in and
out, which changed nothing, I eventually discovered that by pulling the
action out a smidgin (?) the thing would work fine.  So I blocked the
action out a little at the keyframe stop (dag?), which seemed to solve
the problem.

But I never really figured out what was happening here.  Do you know? 
Is there a better way to solve the problem?  I live in the fear that
during Sunday's competition something will happen that will have the
whole world mad at me for all time.  Please advise.  Thanks!

Clyde Hollinger, RPT
Lititz, PA