I think this is really just an impossible question to answer, because somebody could make a case against just about any remotely long-running show based on how you define "major" in clause three. The big three G-T panel shows were pretty much the same from beginning to end, but if you wanted to disqualify them, you could find a way to do so.
Also, I fail to understand how a show that just for fun had an occasional diversion from the norm becomes disqualified. I understand changing to a celebrity format (Hot Potato, Password All-Stars) as being grounds for dismissal, but a "special" week followed by a return to the traditional format doesn't strike me as such a big deal.
Otherwise, your purest answer is probably going to be something that ran thirteen weeks.