We have a Kitchenaid mixer model K5SS. Lately we have found that the upper mixing speeds do not go fast enough. The lower speeds work fine (I adjusted the two upper screws to the control board so that the stir speed is 60 RPM). To increase the upper speeds, I adjusted the lower screw of the control board but cannot get speed #6 to go faster than 120 RPM regardless of how much I turn this lower control board screw. Your thoughts please about why the upper speeds are not fast enough and how to increase the speeds of the upper settings. Thank you in advance!