after going through a lot of work with my bobbin and thread tensions, I found this video that was very helpful:
https://www.youtube.com/watch?v=c65LC6qgm7I
It addresses how to properly load a bobbin on a top-loading (drop-in) machine... apparently my bobbin thread was not properly engaging, which occurred more easily when the machine got older. The result was crooked stitches, especially on the bottom.