Hydro Junkie,
Thank you for that simple explanation. Easy enough for me to undertsatnd without my knowing squat about radios.
UPDATE;
I finally had to change out the receiver from FM to SPCM to stop the uncontrolled fluttering.
I'm glad I had done another ground test today in the driveway to observe the flutter problem reappear! The fluttering occured at only 25 feet away!! (no gazebo and no metal patio furniture this time to blame) Two different FM receivers (JR600 and JR700) with two different #29 crystals didn't fully eliminate it. So finally I put an SPCM JR 770 receiver in the plane and all tests well at 105 feet away now with the radio antenna down.
Moral of the story, this impressed me just how important a good ground check is!!! 
I'll be sure to check it out in the field tomorrow one more time before maidening. I've never noticed intererference issues during the last 15 years I've lived at this house. Evidently, something has changed recently!!