Are you sure the new needle is inserted correctly? I have had a similar problem in the past and it was because I put the flat side of the needle and wrong. Can't remember which way it goes on the 503 - I always have to consult the manual on that!As far as free motion quilting , I always have better luck on the 15 and the 503 by leaving the dogs up and setting the stitch length to zero. IMHO, the 15 does a better job of FMQing.
Good luck and let us know what you find out about the problem.