Many of the APQS Dealers have been trained at the factory to do spa treatments in your home or their studio, depending how involved. They give it a thorough check up and replace parts worn from use. A few examples might be motor brushes, needle bar, rocker arm, hook assembly, re-packing the gear box.
A good indicator of what your machine might need would be how many hours it has been used for actual quilting. It may just need a thorough cleaning. In your manual is a maintenance chart you can follow to keep your machine running smooth for years.