This can result from a number of things. If the Leslie has a bad speaker that is a problem we see a lot. Placement in the room and room acoustics play a big part. You may want to try our tone booster see http://www.organguru.com/276-2/ or our super sub bass system that employs an 18" sub and separate amp. see http://www.organguru.com/super-sub-bass-booster/

This is generally caused by a bad scanner or vibrato switch. This repair should be handled by a skilled technician. We would be glad to help. Just give us a call. 704.798.0299