Check your entire upper thread path starting from the thread spool. Upper thread path issues can cause all manner of problems, including the symptoms you describe. Watch how thread unwinds from the spool as you sew (keep your fingers away from the needle!) You may find it snagging or jerking for some reason. The thread guide hole in the thread stand must be directly above the spool. Some threads are springy and fall off the spool and then get caught under the spool - a thread net may eliminate t