I know Social Security "says" that they look at certain criteria before determining a person capable of working; however, I have never trusted them. As my therapist said, they see the world in black and white. What I think is funny, is that's what I've been told I do and what gives me mental break downs. I wish I had better answers for you.
I don't know that there is much more you can do than get a lawyer; because, they may be able to help you where going up against Social Security on your own is unlikely to be successful. Like most here, I can quote you what the regulations are supposed to be, but I won't tell you that this is really the way SSDI works. What I can say is they work only for the government, and seem to focus heavily on removing people for the smallest of discrepancies. Any one of the reasons given here could be why they are coming down on you, or perhaps there is more to the story, but I wouldn't put it past Social Security to look at less than "gainful" but continual and steady changes in one's work status as evidence that one can and should be working. Wish I had better views, but I just don't trust they have the interest of the mentally disabled person at heart. They get paid based on other considerations.