I've been struggling with a problem making white bread for sandwiches,
following the recipe in Betty Crocker. I've been proofing the yeast at
100F and following the recipe to the letter. I'm having trouble with
the rise.

I've been putting the bowl in a warm water bath for the first rise (it's
winter, and my kitchen's ambient temp is 64). This one goes well; the
dough doubles in an hour and holds finger indentations.

After punching down though, the 2nd rise doesn't go so well. I've been
putting the pans in a 100F oven for this, and the top only rises to
about 3/4 inch over the edge of the 9x5 pan after 75 min. (How can I
get a taller loaf?)

Then- this is the kicker- soon after the bake begins, the top collapses
and I end up with a concave loaf.

Cutting open, it looks like the gas bubbles just under the top were
considerably larger than the rest of the loaf, and have collapsed. That
would explain the concavity. But what causes this? I'm wondering if
the 100F oven was too warm for the 2nd rise.

Thanks for any tips!