Besides the slotted washer... When you are tightening the lock nut... make sure its not turning the cone. I've used a hook pick with some success to hold the cone in place while tightening. (Prolly noticed that its a freaking tight squeeze for wrenches!)

Lastly, pull the balls out and check for any that have pits... if you replace one, replace them all on that side. Then see if cup has any dings or scuffs.

Good luck with it... Sylvans are smoooooooth when set up right.