I have the same machine but have not had this problem except when trying to sew 2 triangles together and the points get pulled down. Your repairman is incorrect: if you have a single hole needle you can't move your needle to adjust the seam width. I don't have any answers. Have you cleaned out the bobbin area and around it? Sometimes a thread gets caught down in there. Good luck and check with another repairman or HV again. (Sorry I'm not much help)