I made a 9x12 pan of cinnamon buns and I've got a problem.  I have made this recipe several times, and the buns taste Great!  In making them the dough feels great, it rises nicely, the buns roll out and cut just fine, and after the second rise they look simply awesome!  Even when they come out of the oven they look (and smell) wonderful.  However, as they cool, the center two buns "fall".  I make 12 buns in my 9x12 metal cake pan (3 rows of 4 buns).  The outer buns stay nice and tall on the side next to the pan wall, but slope toward the center of the pan.  The center two buns are maybe half the height of the outer edge, and maybe half as tall as they were when they came out of the oven.

I suspect my problem has to do with oven temperature and time, but I follow the recipe I have religiously (375' oven; 18-20 minutes).  After 20 minutes the buns are quite brown on top, and I would hesitate to leave them in longer as I fear they would become too dark.  Any suggestions???