Sound cuts in and out when connected to an amplifier

Sound cuts in and out when connected to an amplifier, and expression pedal.

I am connecting with the headphone outlet to an expression pedal and into Behringer Ultratone K900FX keyboard amplifier. I have tried connecting to 2 other amps, and removed the expression pedal, but it still does the same thing.

My Hammond M-Solo works fine in the Behringer.