According to the cake books I have looked at, cakes rise and then fall in the middle if you have too much leavening (baking soda) or too little "structure" in the cake batter. Structure is caused by the gluten in the flour. You can strengthen structure by using more flour, or using flour with a higher gluten (protein) content, or beating the batter longer. Adding more fat to the batter weakens the structure.

So, I have a couple of thoughts about what could have gone wrong.

1) Sour cream in the USA is 18 to 20% butter fat. If you used a sour cream that had a much higher fat content, that might have weakened the structure.

2) "1 cup of flour" is such an inaccurate way of measuring! All the other parts of the world are more sensible and measure by weight. If possible, look in the "about ingredients" or the "about measuring" sections of each cookbook to find out what type of flour they use and how they measure it.

Here are some weights from my favorite book for understanding how to bake a cake, Rose Levy Beranbaum's "The Cake Bible." She has a blog site, by the way -- it is http://www.realbakingwithrose.com/

1 cup of sifted cake flour = 100 gm
1 cup of sifted bleached all-purpose flour = 114 gm
1 cup of lightly spooned bleached all-purpose flour = 121 gm
1 cup of dip-and-sweep bleached all-purpose flour = 142 gm
1 cup of lightly spooned UNbleached all-purpose flour = 130 gm
1 cup of dip-and-sweep UNbleached all-purpose flour = 148 gm

Nice blog.
:-)
http://www.iniww.com

Mir kommt die halbe Tasse Wasser am Schluss etwas heftig vor, stattdessen hätte ich eher die Sahne-Menge verdoppelt. Ich hasse das auch, wenn Rezepte nichts werden... (grummel ;-))