Quick update. Yesterday I recleaned the bobbin assembly on the 503. It wasn’t very dirty. So I changed thread (back to a 50 weight) and did a sampler with all 22 cams. Not one catch/break. I have been using an 80/12 needle but had changed to a 40 weight thread when the problems occurred. I had no trouble with this combination on my 201 machine. I love that machine! But sometimes I need more than a straight stitch. So the problem seems to be the thread/needle combo.
Thank you all for your input.