I am making a pain de champagne. I use a starter, I knead it for 10 min, let it rise for 2 hrs and then shape and proof for 1 1/2 hrs in a bread basket (before that I didn't use a bread basket and it just proofed wide). Now with the basket it has a nice shape, but as soon as I take it out it starts to sink. I've gotten good at throwing it in oven before it sinks too much. What am I doing wrong that it won't retain it's shape?

thank you for your help!