Only if I overfill the bobbin and the thread slips out of the slot in the bobbin holder. Have you 1) cleaned the tension disks with crochet cotton or dental floss 2)cleaned out the lint in the bobbin area? A dirty machine will do strange things. You have to remove the two screws in the needleplate to really clean it underneath. Be sure to put the little hook back between the groove in the plate before tightening the screws. Do you have a manual - good cleaning instructions there?