If you're using, say, all whole wheat flour, expect brickbats.
I make 100% whole wheat bread all the time with no issues.
My oven allows me to set it to 100°F which is perfect for proofing yeast and raising bread. But I used to make bread on laundry day - the top of the dryer was the perfect temperature.
I proof my yeast - 1/3 cup warm tap water in a coffee mug, 1 tbsp sugar, stirred in, add 1 tbsp or 1 package of dry active yeast and place in the 100°F oven. Let the yeast grow until it is almost to the top of the mug. Add to your recipe. Mix everything and knead it then put it in a bowl in the oven to rise until double in bulk, about 1 hour. Once it has risen the first time, turn it out, knead it to remove large air bubbles then divide into the number of loaves in the appropriate sized loaf pans that your recipe calls for. Shape and let rise until just above the top of the pans - my method takes about 35 minutes. Remove the bread from the oven, preheat to the required temperature and put the bread back in once the temperature is up to the required level.
Things that may be contributing: you are making more loaves than the recipe calls for or the pans are larger than the recipe calls for and the bread is rising too much before baking; you aren't letting the oven heat to the correct temperature before putting the bread in, which causes it to rise more while the oven is heating up, then causing it to collapse because it is too high; your yeast might be old or inactive.
ETA: Just saw your recipe - mine is even more simple:
5 cups flour (all purpose or whole wheat or combination of both - just adjust the rising time to get the double in bulk rises)
1/4 cup margerine or butter
1.5 tbsp salt
Yeast from above
1 2/3 cups ice water
Mix the flour, butter, salt together and add the yeast. Slowly pour in the water (I use a kitchenaid mixer. Mom always used her Cuisenart food processor with the plastic blade.) Knead for 3 to 5 minutes in the mixer. Follow the rising procedure above. My bread pans are old head cheese pans - the sides are higher and they make a really nice loaf of bread. They probably help with the support, too, because the loaf doesn't rise a lot above the sides of the pan. Bake for 30 minutes at 350°F.