Larry,
I don't know for sure, but the only thing that I can think of would be that when playing the Cordovox with the accordian and organ sounds combined, one is generated by reeds and one by electronics. The point being that they would hardly ever be in exact tune and would generate a low beat that would result in an amplitude tremolo.
You might try setting up an accordian voice on Rt-1 and a drawbar voice on Rt-2. Then experiment with detuning one or the other (or both in opposite directions). Then try adding some LFO modulation (Ampltude on page 4 of the LFO settings)to compound the effect on the accordian voice.
I haven't tried this myself but it is what comes to mind.
Good luck and Regards,
Walt