They get wear on the bearings causing wobble. This fluctuates your DC output.
Sometimes it is just a bad fitting coupling or broken coupling. Put your meter across the tach leads and check for AC voltage. Any fluctuation here will be seen and felt by the drive. It will nevery be exactly zero but anything greater than about 300mv is potentially causing vibration. If you have one of the better tachs ie Radio Energie it helps. Sometimes guys put a load resister across the tach to help clean it up. Some have four brushes. Remember the tach is nothing more than a little generator.