Yes, the belts can be too tight. That causes all kinds of trouble. FWs are finicky little things. The bobbin is the usual culprit, if the needle is correct. Thread can become wrapped around the hook and in the bobbin case. Eliminate all ravelings. Yes, you may have to take the bobbin hook out or at least take the plate off, so you can check for ravelings. You said you did, but sometimes they hide. It has happened to me and some other at gild. Haven't had that trouble with my 301 though.