I agree with the above...sounds like the spindle is bent...this is hard to detect w/o taking it out...but one way to test is to put an allen wrench on one end and hold the blade holder on the other end...if the blade holder moves up and while turning the spindle that is the problem....I had this problem on one of my 30 size xcells...drove me up the wall til I figured it out.....Also a too high head speed could be the cause.

Ken