Your problem is almost certainly a bad commutator segment in the motor. In plain English that means you either live with it not starting by itself 1 time in 10 (on average) or you have someone install a used motor from Ebay. I have done several jobs like this for customers and if you are handy with tools this is also a job you can do yourself. Good luck with it!
David Werther
www.quilters-connection.com