I took it all apart again, including the bobbin case which had old thread under the tension leaf and the upper tension assembly, tried a bobbin case from a different 15, cotton swabs, oil, toothpicks, and switched out needles a couple of times, changed thread 3 times, back to the first bobbin case, and it sewed just fine. I just hope I never run out of the thread that worked!
Thanks for everyone's input.