I've not had that problem, but I think filing the hook is asking for problems that can't be fixed. It might mess up the timing.
Being Labor Day weekend, we may not get a lot of good input until after the weekend. After what WIChix mentioned, my thought would be cleaning the bobbin area very well. I had some problems with thread knots that ended up with the machine locked up, and were resolved with cleaning.
I followed this
youtube video. The 401 is basically the same as your 500. It doesn't cost much and doesn't take long and it might resolve your issue.
bkay