Hi Emily & Toni,
I've had similar problems with my 300E. There were 2 things I found:
1. As I use sticky stab. and sticky spray a lot, some of that gunk transfers itself to the hook, which then hangs up the thread as it passes over the bobbin.
2. The needle can get bent very easily, and it only takes a slight bend to make the hook miss the top thread.
The solution is to remove the needle plate, the bobbin and the bobbin case. Use a small lint brush to pick-up and remove any lint/fluff that gets trapped inside the cavity. Don't try to 'blow' it out as this can drive the fluff deeper into the machine, causing more problems!
After that get a tissue and cotton bud/Q-Tip wet with methylated spirit/de-natured alcohol and wipe over the hook mechanism, especially down into the bottom of the hook cutout and the small lip that the bobbin case sits in. (Turn the machine by hand to access all of the hooks' surface.)
Wipe over the bobbin case surfaces (pay close attention to the underside of the lip where the case sits in the hook mechanism) and also in the hole and the underside of the needle plate.
Refit the bobbin case but leave the bobbin & needle plate off for now. While slowly turning the machine by hand, put a very small
bead of sewing machine oil in the race where the bobbin case sits in the hook. Seems to make the case ride a lot smoother during operation. Clean off any excess oil as it may transfer to the thread during operation. You should not be able to see any oil on the surfaces.
Next, run a top thread through a NEW needle, then turn the machine by hand and watch the top thread as it's picked up by the hook and turned around over the bobbin case. (You'll also see how close-tolerance this is and why a bent needle can mess the whole thing up!) Make sure you're not getting any hang-ups or sticky bits and that the thread moves smoothly through its path. It should release from the hook very easily. If all's OK, refit the needle plate and bobbin, re-thread the machine and do a test sew to see if it works OK.
BTW, Emily, I enjoyed your blog, especially the link to Super Cook. If you liked that, have a look at this:
The Living Cookbook.
Seems to be a great resource and well supported.
Steve in Au.