At long last I believe I have solved the mystery of why my 500A was so frequently "hanging up" while stitching. It was truly disastrous -- sometimes the spool of thread would actually rocket (hehe) off the spool pin and strike me. I thought by replacing the hook, retiming, and frequently oiling the bobbin case/hook race I had solved the problem, but lately it became so bad I would get about 10-20 stitches off before having to set things back in order and pull the work from the throat plate, breaking threads that had gotten hung up. I also thought it was due to Gutermann cross-wound thread (which still gets way too twisty for me to think is ok, but I digress).
I think I've finally diagnosed the problem, and I believe there's really only one way to fix it, which I did.
The thread was catching on the corner of the "hook positioning finger," out of which usually sticks a lint-clearing pin. I removed that pin, and smoothed the area it came out of, and even plugged the hole that contained the pin. This still did not fix the issue.
Here is a photo of me catching the problem having just occurred, followed by a photo of me releasing the thread. A loop of the needle thread was repeatedly catching on this corner of the hook positioning finger:
When I realized that that corner had to go, I made it so. I spent the morning with a Dremel grinding it down -- luckily I had extras to play with. Here are photos of the hook positioning finger that was initially in the machine (you can see the former lint clearing pin hole that I stopped up with the end of a plastic toothpick), and the new and improved hook positioning finger that will no longer catch the needle thread. They are exactly the same part number and size -- some trick of the camera makes one look larger in one photo.
I reinstalled the hook and finger, retimed the hook (thankfully because I "dotted" the gear teeth re-meshing them was simple and the hook didn't actually need retiming), and now it sews perfectly, and the thread no longer hangs up -- Gutermann cross-winding nothwithstanding.
I hope this is helpful for those of you with a 400 or 500 series that is exhibiting this behavior. I would also gladly offer this service to anyone who wants to send me their hook positioning fingers, for the cost of postage.
Matt