I'm finding a few people report that they've seen a really dark film base from over development, high heat, or too much agitation here's one person and it's also in a guide from FSU—the latter is one of the best things I've read on the subject. I called Kodak, and one of their techs also suspected overdevelopment. I'm thinking I'll double check my developer mixture, go easier on the agitation, and make sure my thermometer is calibrated.
I'll update when I learn more. Thanks, folks.