Check for a stripped servo arm / spline, this would produce the behavior you described as well. Place a light load on the servos by hand and raise/lower the collective slowly looking for any odd behavior.
I am thinking servo issue based on your 20-30ft comment initially, you can try lowering the setup->swashplate->outputs->frequency to 70hz and see if it helps. Maybe the servos just do not like 333hz.
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