Hi Imo, Check that the feathering ( spindle) shaft between the blade grips is not bent, if not make sure that the main shaft is straight as well. These are common causes along with unbalanced blades that will cause unwanted vibes. You may have to remove them to check them properly, any slight bend will cause problems. Usually spinning the feathering shaft with a alllen wrench or such while it is still installed will show any bends with wobble in the blade grips. Inspecting the main shaft sometimes requires removal to check for them. Hope this helps you out.