Dick,  I've found that oscillations happen when the bleed to the sleeve
spill pneumatic is missing or too large.

Look at your schematic.  Anything that will cause the sleeve pneumatic
to overshoot will also cause oscillation.  The bleed dampens the pump
pulses, and prevents the sleeve from overshooting with each pulse by
slowing it down.  That allows the sleeve to react slowly and out of
time with the pump, so it won't oscillate.

Craig Brougher