First step if you can't get reliable operation is to check the flywheel magnet ring. That's a fairly cheap repair if that's the issue. They're glued to the flywheel and frequently come loose. If they're not yet broken, you can simply glue them back on.
Stop running them until you check as when the magnet ring breaks, then you have to get a new magnet ring, pull the flywheel to glue it on, and then replace the flywheel. This is clearly a lot more work than if the ring is just loose.
