This happened to me, too. It was the encoders. One of them needed to be adjusted,even after cleaning tracks and wheels. It literally was not making contact with the rail because it was too high. It was difficult to see that it was not making contact. I'm not at my machine now, but I think there is a little screw on the encoder assembly that is used to adjust the encoder up or down a bit.
Also, make sure all connecters are in snugly. They do vibrate out of place as you quilt.