I just baked my 1st 14" round layer. It fell in the middle. I thought I was doing everything right. My oven temp is right on. I baked at 325 degrees, used 3 flower nails in the center, and didn't open the oven door. The recipe I used was one I got from here and have used it in smaller pans with great success. It is 1 cake mix, 1C. sour cream, 1 sm box instant pudding, 4 eggs, 1/2C. water, 1/3C. oil, 1 tsp. vanilla. The pan I used is the Wilton Performance Pan. I think the cake will be ok, I will just have to level it. I very disappointed that it fell! 
Any suggestions as to what I should do when I bake the next layer to keep it from sinking in the middle?
That's a good thought, but no. The cake fell long before it came out of the oven! I was surprised that it baked as quickly as it did though. Took a little less than an hour, but the toothpick in the center came out clean. Maybe I shouldn't have used 3 nails?
I have the same problem when I make 14", 16" and I have yet to attempt an 18" cake. But what i have learned is that cakes cook from the heat source and then towards the middle. I have developed a trick for larger cakes! Use a 'thingy' (not sure of what it is called) that you make tube cakes with. Wrap abot 50 to 75 pennies in foil and set it inside the center of the "thingy' and set that in the center of your pan and then pour in your batter. I know, your cake will have a hole in the middle, you can fill that. But the heat will go from the middle and finish cooking your cake. Commercial kitchens do not have to worry about that because they have the ovens that create a heat source great enough to surround the cake to cook it thourooghly and evenly.
I bake large tiers a lot. I never use a heat core or nails. I bake at 325 degrees for as long as it takes...usually 50 minutes to over an hour. I've never had one fall. When I place the batter in the pan, I always bang the pan on the table to remove air bubbles. I make sure the batter is level and slap that puppy in the oven. That's it.
I agree with the others, it sounds like a combination of the cake not being completely done and maybe too many nails. I've never used them.
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