Okay 1/3 top and 2/3 bottom is a good general STARTING point, but it isn't right for everyone. Embochures are different for different people. Different players have different physiologies.

Sometimes ones embochure might actually change over time...for instance, mine moved over a period of years because my teeth shifted due to problems with the way my wisdom teeth grew in. In my case this wasn't a problem, but it did happen.

What concerns me about the original poster's situation is that it doesn't sound like his embochure has shifted, but rather, that his embochure is moving around, outside of his control. This is not good. Rowuk and Grier suggest a stable practice routine. I concur. I also think that if the OP can get to a good teacher, it would be a wise move.

Typically, when I see students with these kind of problems, it's because they have developed some bad habits, and they typically don't have a good practice routine that keeps their fundemental playing milieu (embochure, breathing, etc.) stable.