Thanks, bkay, that was a fascinating read. I cleaned up the bobbin case area, removing a couple of thread bits that were stuck down there. No change. I took off the top cover plate and looked for anything that was obviously broken, but all seems good. I oiled things up top again, and while she will work, she doesn't sound happy about it. I'm going to put her in front of the heat register overnight and try her again in the morning.
There's a special place in my heart for the 401A. I'd hate to give up on one of these beauties.