Dave Smith Instruments SYNTHESIZERS

hello!  this is my first post here...

so an annoying problem has popped up a few times with my desktop 
evolver:

When I've tried to create 808-style kick drums and deep dub-style 
bass patches using the two analog oscillators tuned to the same 
pitch (and with the amp output in "mono" rather than "stereo" mode), 
I get a slow sweeping phase cancellation that causes the volume of 
the sound to fluctuate in a roughly sine-wave-like pattern.  So as I 
keep triggering the sound over the course of say 30 seconds it will 
go through maybe one cycle of quiet and weak to full and deep.  This 
is annoying because I want to create some deep, low speaker-rattling 
kind of sounds.  And I know it's phase cancellation because it 
doesn't happen if I disconnect either output from my mixer.  And 
this is happening on sounds with no detuning, mod, filter split, or 
anything else I can think of that would cause phasing.  I guess the 
problem could be in my mixing board or monitor setup, but i've never 
noticed it with other low-pitched sources.  It seems that the 
problem is occuring in the summing of the left and right signal 
paths of the evolver.

Any ideas or solution?

Thanks!