i have singer sewing machine with under stitching looping all the time . i think its the timing

stitch, July 2010

You must make sure you have the threading and the tensions set properly first of all, normally it will only need to be retime if the machiune is missing stitches, if you type in my email address as you say it and not as you see it I will attach instructions for timing,
Ken

drivewithken at blueyonder dot co dot uk, July 2010