I had to deal recently with a similar problem in a Duo-Art grand.
It turned out one of the pump bellows was leaking badly -- the cloth
had come unglued on one side.  There was a big imbalance problem in the
pump and the grand shook about 5 cm to either side.  After re-gluing
the cloth the shaking is down to about half a centimeter.

Check the pump bellows for airtightness first, I'd say!

Regards,
Niels Berkers
Arnhem, The Netherlands