I have never seen that. but I have experinced that they either "hang" in the curve, not wanting to come down from about 20 degrees of advance to idle (10) Or the other way around wouldnt go past about 22 degrees on the way up. In all cases I had to replace the entire rotorhead then things began to work normal again.
Once they work, they are about as good as an MSD. But it does take some fiddling around.
But hey, its still better than the 1-2-3 dizzy
T