Try using a different thread. Sometimes a spool will have an invisible defect and just won't sew right. I have doubts it is the bobbin or timing. Timing would mean it won't sew at all, and the same thing for the hook. Is there somewhere along the thread path that it is getting caught?
Of course you have already tried the standard things - new needle, cleaned out bobbin case, oiled bobbin area (if applicable for your machine), retreaded ...
Pam