I've spent a lot of time retiming 301's (which are basically the same thing in this instance). They can do a perfect stitch and piece great, but when I try to FMQ (fast and needs to be dead on) I would get skipping. Upon checking the timing, it would be off a bit. NOt enough to give me fits when sewing a seam and you would think everything was fine. But, fire it up and do FMQing and timing would be off just enough, that the hook wasn't catching the thread EVERY revolution. This FW's timing may be close but off just enough that high speeds catch the problem. If the bobbin assembly has been removed like you mentioned, I would suspect the timing as my #1 guess.