It turns out that some of my Schacht bobbins (the high speed ones) are a little defective. One wasn't tooled properly. Instead of assuming that, I spent the better part of friday night beating myself up, and restringing my driveband.
(Because CLEARLY if the driveband has been working properly for everything else, that must be what the problem is when the only variable that has changed is a bobbin.)
So, we restring the driveband. No luck. I oil. No luck. I stomp my feet. Andrew suggests working on something else until I can ask Sandi if it's a break-in issue, or a *gasp* BAD BOBBIN.
So, Saturday morning, I take the Bad Bobbin, another stiff bobbin from that batch, the Control Bobbin (which works just swimmingly), and the appropriate high speed whorl. Sandi confirms my suspicions, and lets me know that she'll call Schacht to see what their solution to the Bad Bobbin is. I buy two more High Speed Bobbins, so that I can do this: