<<Disclaimer: Verify this information before applying it to your situation.>>

Cave-ins are usually caused by too much yeast. The bread rose and then
fell. Try decreasing the yeast, 1/2 teaspoon each time until it comes
out right.

Check the liquid, too. Start with half that required in the recipe, and
then add more as needed to get the "dough" to about wheat-muffin
stiffness (not as thin as a batter, not as stiff as wheat-bread dough).
It should just barely be able to heap up when kneaded and then left to
rest.
--
Linda Blanchard
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http://www2.basinlink.com/us/mindpla/main.htm  <==celiacs and sf
Midland TX USA