That's odd, I never have any of those problems with my juicy 7's and I am about due for new rotors for them being that my rotors are both wobbling and they are losing the plated finish on them and the only time I get any sound is when it's wet out. Sounds like the only way to find out is to get a different brand brake and if it still warbles it means that it isn't the brakes warbling.
Question, when you assembled the bike, did you remember to face the disc brake mounts on the frame, there is a slight chance that non faced disc brake mounts are the culprit.
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Appreciate the old bikes more than the new.