I am guessing that you are hitting a resonant frequency of some sort. All structures will vibrate at a certain frequency (think of a tuning fork). If you have an out of balance prop spinning at the same RPM as the natural (or resonant) frequency of your airframe, the vibration will be amplified. Even if the prop is balanced, your airframe will tend to vibrate more at that speed.

This behavior is also found in full scale aircraft which is why aircraft often have certain engine operating speeds that they must avoid or pass through as quickly as possible (I started out my career - more years ago than I care to admit - working for Hamilton Standard on aircraft designing/testing full scale propeller systems).

You could try soft mounts to isolate/damp the vibration, but definitely avoid prolonged operation at that particular speed.

Al