Problems with Pound Cakes

Hi. I'm having a difficult time trying to figure out just exactly what I'm doing wrong each time I bake a pound cake. I follow the directions exactly as written, the oven temps correctly, etc. However, every time I try to bake one, it "explodes" in the oven. It overflows the bundt pan every time...no matter which one I use. I have bought several different ones, thinking that I just don't have one the right size, but I've bought the biggest one I can find, and it still overflows. 

Please...someone help me figure out what is going on with this!!!

3 cups plain flour

5 eggs

3 cups sugar

2 sticks butter

1/2 cup shortening

1 1/2 tsp vanilla

1/2 cup cocoa powder

1/8 tsp salt

1 cup milk

1/2 tsp baking powder

Preheat oven to 325. Bring all cold items to room temperature. Cream together butter, sugar, and shortening. Add eggs one at a time, beating well between each one. Add vanilla. Combine all dry ingredients. Add a little bit at a time, alternating with milk, mixing well between additions. Grease and flour bundt pan well. Pour cake mixture into pan evenly. Bake at 325 for 1 1/2 hours. Do not open oven during baking time. If cake has not pulled away from sides of pan after 1 1/2 hours, leave in an additional 10-15 minutes. Let cool on rack 30 mins before removing from pan.

This is the same recipe that my husband's grandmother has used for years with no issue. She can't seem to figure out what I'm doing wrong. Also note the the cake overflows within the first 20-30 minutes of being in the oven.