Marions ... there are two types of needle plates (the removable plate that sits under the presser foot and over the bobbin area). One type has a single hole in the middle, the other has a rectangle or U shape hole. The single hole place is ONLY for straight stitching as the needle would "crash" into the plate if you tried to zig-zag. The other plate with a longer U or rectangle shape allows the needle to perform in multiple positions along the horizontal (i.e. zig zag).
Note: If you are like me and you adjust your needle position for 1/4" seams, even though you are doing a straight stitch, you CANNOT use the single stitch plate.
If this is happening when you are chain stitching I suspect the problem is elsewhere though. There might be a burr on your needle, or a burr on your bobbin or bobbin case that is catching the thread every now and then and tugging the fabric under. You might also have a small piece of thread stuck *somewhere* in the bobbin area. I suspect the problem is most likely in the bobbin area ... something is catching and holding that thread for a split second. Possibly even a burr on the underside of your needle plate.