Is it actually the bobbin thread that loops or is the loop appearing on the underside of the fabric as you stitch? If it is a loop under the fabric, that's usually an indication it is an upper thread issue. Thread irregularities that appear on the topside of the fabric are generally bobbin related.
First thing to try is installing a brand new needle. Verify it is the correct needle for the fabric and thread being used.
All About Needles
Sometimes, the fabric being stitched will "grab" the thread and hold it for a second before releasing and will cause a loop. In this case, a needle with a larger scarf and eye may be helpful.
You might also open the upper tension as much as possible, RAISE the presser foot and run a piece of UNwaxed dental floss through the tension disk a few times to clear out any gunk that may be stuck there. A piece of pearl cotton dipped in rubbing alcohol and squeezed moist/dry will also work.
Remove the upper thread. RAISE the presser foot and rethread the top thread from the beginning. Use only fresh, good quality thread--NO old or bargain bin variety! Try using the same thread in the top and bobbin. Make sure the thread is firmly seated in the tension disk and the thread path is correct. Check that there are no burrs or snags in the thread path that could be catching the upper thread. Especially, check that the thread is not catching on the spool "slit" that holds the thread when in storage. Every time the thread navigates around the thread spool, it could be catching on that slit just enough to cause a loop.
Try changing the bobbin. There could be something amiss with the current bobbin--could be nicked or could even be the wrong bobbin for the machine. Also, make sure the bobbin is loaded and turning the correct direction in the machine.
Set the upper tension to the midway point. This is usually the starting point, although it will probably require some tweaking. Test your stitch and adjust the tension so the upper and bobbin threads meet in the middle of the fabric. Neither thread should dominate one side of the fabric or the other.
Understanding Thread Tension Threads
Last thing to try is a different presser foot.
If you still have the intermittent loop, you might want to take it for service. Could be a problem within the tension disk.