It's a cheapie Singer Stylist 7258. I've turned it off and on so many times as I've been troubleshooting the problem here and there for a couple days now, and that made no difference. Tonight I wound a new bobbin and put the aurifil on the horizontal spool pin rather than the vertical spool pin. Seems to have solved the problem for now. The stitch looks good with the upper tension dial set where it was before this issue randomly appeared.
I'm thinking maybe the presser foot does have something to do with it, but putting the thread on the horizontal spool pin is working for now.