I had to check since I think we went 5 pages of diagnosis and I had almost filmed a video on checking timing before we figured out the needle.
What did you take off the machine to put on the Aurifil?
Those orange cogged belts sound awful. I don't like them either.
What's your bobbin tension set to? If you count the number of turns before you hit full tight, you can put it back to what it started at. There are only 3 things I can think of that would let that thread pop out of the bobbin tension spring.
1. The tension spring is damaged.
2. The tension is set way too low.
3. The bobbin is in with the thread pulling off the bobbin clockwise - it should come off widdershins - and possibly one of the above two problems to a smaller degree.
What happens if you use that bobbin case in your other featherweight? Does it jump out there too?
ETA: if she's grumpy, it might also help you to try a thread stand for the cross wound thread. I find my little Ronnie (FW) likes her thread delivered from the thread stand better. It could have to do with the cross wound threads wanting to come off the top of the spool and the jerk and stop of the thread when on the featherweight might tighten a little here and there and cause no space for the hook to grab the thread?