Technical help? (My bobbin casing hates me.)

[sapote]

Okay, so I made that same old sewing mistake where you let a lightweight fabric go down below the needle plate by accident and it jams up. And just like old times (before I last got my machine serviced) this has made my bobbin casing start to jam on every fourth stitch. I've taken all the various covers off so that I can see what's happening, and it looks like the spool thread from the top is circling around the ... round metal thingie the removable plastic bobbin housing sits in just fine for four goes, and then on the fifth go it doesn't release the thread and it just catches, so the next time it goes around the whole thing gums up and pulls the little plastic housing the bobbin sits in right out of the ... rotary metal thingie. At which point everything gums up and makes horrible noises.

I have a Brother computerized machine, but I've been testing it out manually with the machine power off and it's definitely a mechanical error. Help?

(I am so frustrated, I was finishing the edge on the waistband facing and then it was zipper and then it was done with a whole from-scratch project.)