Sew, the straight stitch works, and you have the hook timing set after replacing the gear. (?) Now to check the zigzag cam timing and bight adjustment. First make sure there is no end play in the main shaft; push/pull on the fly wheel and make sure there is no play here. Now slowly turn the wheel by hand with zigzag and observe the needle position left/right as it enters the needle plate. It should be equally spaced on either side of the slot, and should only be changing position when the needle is above the plate. See if you can determine what/where is causing the binding...