Pressurization Issues-P337G

[Joes]

Cabin pressurization works fine until you get 1.0 psi pressure and then the cabin rate fluctuates from a plus to minus 1000 fpm indication, hard on the ears.
The shop inspected the outflow and safety pressurization valves, no defects found. They disconnected the controller line at the outflow valve and flew the a/c. The cabin pressurized to 3.2psi with no fluctuations. The shop seem to think the controller has some issues. Any thoughts or information where i can sent the controller to be bench tested/repaired.Thanks Joe