The bit I always struggled with sourdoughs is the baking part. It would just never bake correctly and end up either exploding (large pocket of air) or be some kind of subterraneous sponge cake. I've never had this issue with baking regular yeast bread.
Sounds like you might be over proving it. If you’re at a high altitude, or live in a warm place, or it just keeps happening, try knocking half an hour off your proving time, and keep going until it starts getting worse.
