While hanging out at the Tunnel Creek aid station during the Tahoe race, I talked to a podiatrist who was volunteering her time for the race. She was very helpful and interesting in hearing about my problem. I told her what had been going on - the history of my problems, the negative x-rays and MRI. She described something called "Cuboid Syndrome" and after talking to her and hearing about symptoms, etc, it seems to be the most likely cause of my problems. At least from what I have hear so far.
It's even possible that this problem has been going on long enough to have been diagnosed as that metatarsal crack I had at the beginning of the year. That was diagnosed by one doctor via x-ray, but another doctor said he saw no evidence of that on my x-ray. So, it IS possible I never had a crack and this cuboid problem has been the culprit. Whether or not it was the culprit at the beginning of the year, I'm going with that for my current problem and I'm planning on getting into my doctor and/or a physical therapist and get this thing taken care of.