Regarding the following problem, I have two suggestions.

    > It always plays normally when I play (via pedaling) a single roll,
    > but when I play a large three-piece roll (for example a QRS XP
    > roll), it comes to a complete stop for some reason, usually in
    > the middle of the third and final movement.  With continued
    > pedaling, it plays normally again for a short time and then again
    > comes to a complete stop.

One, check the tension on the forward roll brake.  Also check for
contamination of the brake pad or friction brake.

Two, check the adjustment of the air motor governor.  See:

    http://www.player-care.com/governor.html 

Here are the reasons for my suggestions.  Fairly often the increased
load of a long-playing roll on an air motor will make the motor work
harder.  Also, long-playing rolls are generally played at a slower
tempo.  The combination of these two factors can cause the air motor
governor to collapse more than when playing a lighter regular roll at a
faster tempo.  If the governor isn't adjusted correctly, it can collapse
so far that it turns off the vacuum to the motor, and the motor will
stop.

If the tension on the forward roll brake is too strong or if the brake
pad is contaminated with oil or grease, it is continually applying
excessive drag.  Sometimes the drag will become so great that the roll
stops.  However, when this happens, the take-up spool keeps turning,
which tightens the paper that's already on the take-up spool.  Once the
paper on the spool is tight enough to overcome the drag presented by the
roll brake, the roll starts moving again.  Corrective action is to
remove the contaminated pad, clean the brake drum, and replace the pad.

Lastly, there is one other possible problem.  Although rather uncommon,
it's possible that the drive pin (on the right side of the take-up
spool) is slightly loose.  Here again, the increased load of the heavier
long-playing roll, coupled with the drag from the forward roll brake can
cause the drive pin to slip (or spin freely).  In this instance, it's
relatively easy to identify the problem because the air motor and
transmission keep moving even though the roll has stopped.  To fix the
problem, apply a drop of Krazy Glue around the base of the pin and let
it dry thoroughly.

Musically,
John A. Tuttle
Brick, NJ