Do you have a picture of what you are describing? From the sounds of it, it could be the edges have baked quicker than the center (which is what happens). You can slow down the process of the edges baking using baking strips (forget the actual name, but they are water soaked strips you wrap around a cake) and you can speed up the center baking using heating rods/heating core. I use heating rods for 10 inch and larger and may use a couple.
