I'll definitely go and check my bobbin tension. I'll do that "yo yo" test. I thought a tension problem would be where there is a straight line of thread either on the top or bottom. That's what my manual and the web says. So tension problems cause these funny stitches too?
I don't get my machine serviced. I don't have a OSMG. I try to take care of it myself. I have the service manual but I'm a rank amateur.