Aha! I adore YouTube in small doses. I was looking for information on Endless Embroidery and I found an incredible video from Bernina Tampa on how to manage that spring in the bobbin case that gave me so much trouble.
The trick isn't just to pop it into place. I'd been lucky with that. When I removed it to photograph the darned thing, I found that I also had to press the spring into place all around the edge of the spring. I used the tip of my Purple Thang and it worked perfectly. It still didn't take lots of pressure. About what I'd use when writing with a pencil was good. The test is to pull on the thread and feel that the tension is steady and doesn't get tight and then loose again or that it isn't horribly tight and doesn't get loose at all. If the spring is really out of place, you can't load the bobbin into the case in the first place.
If you look through the videos that are listed off to the right, you can find quite a few videos on Bernina mechanical and digital functions. I am NOT suggesting that you follow the directions and attempt to become your own Bernina tech. But knowing what's going on inside your machine never hurts.
I continue to learn, a little bit at a time, about this machine. Later today, I'm off to another class on Embroidery with Big Mama B. Details to follow!