What happened to my cake? I have used the same pound cake recipe in the same pan for years.  The last two times I made it; the cake was a hot mess.  The cake overflowed the pan onto my my freshly cleaned oven.  It was very giggly.  I continued to cook it, and when finally done, it sank.  

6 large eggs
1 cup butter
1 cub buttermilk
3 cups all purpose flour
1.2 teaspoon baking soda
2 1.4 cups sugar
2 teaspoons grated lemon peel
1 tablespoon lemon juice
1 teaspoon vanilla


Beat butter, then add sugar, add eggs one at a time and beat for one minute after each/scrap sides, add lemon peel, juice, and vanilla, add flour and buttermilk alternately, start and end with flour.  Bake 55-60 minutes at 350 degrees.
Over-rising and then sinking sound very much like a cake made with too much leavening. 

Since you cake is made with the creaming method, and baking soda, there are two sources of leavening:


The baking soda, as a chemical leavener, will react with the acid from the lemon juice and butter milk.  Have you changed brands or amounts of any of these items that might change the acid ratio?
Creaming the sugar with the butter causes the sharp edges of he sugar to cut air bubbles into the solid butter phase, which serve as the seeds of the leavening.  Has anything changed here, such as starting to use a stand mixture?