Ah, I just read the answer to this in The Fat Duck CookbookThe Fat Duck Cookbook. Foams collapse when the water drains out of the bubbles. Ingredients like gums that thicken water slow this, and those that thin water, like alcohol speed it up. Adding the alcohol just before serving (with just a bit more whipping) can help. You could also try adding say 1/8 teaspoon of xanthan gum dissolved in the cream to stabilize it a bit.