I dropped mine off at the shop today and they're going to call me in a few days to let me know. I agree that timing is going to be something that is better left to the shop to figure out. Although the cause might be simple, on the off chance that it is more complicated, I'd rather have someone who knows what they're doing to look at it! I just don't trust myself at this point because nothing seems suspicious — no damaged bobbin case, etc.
Thanks, everyone, for sharing your thoughts!