Ok, I have been making bread for some time now, and it has always seemed to work, but now I have developed a problem.  Really don't think that the mix has anything to do with it, so I will leave it out for now. 

I usually use 2 cups of WW flour, then use AP unbleached for the rest.  I put the yeast in with the dry ingredients, then heat the liquids to 125-130 degrees, then add to the dry ingredients.  I proof the bread in the oven, after I set the oven to 400 degrees for one minute, then turn it off.  I cover it with plastic and a towel, and let it rise until doubled.  I then seperate into 2 loaves then place in bread pans, cover and put back into the oven to raise.  This has worked well for the last 20 to 30 loaves that I have done, up until the last 2 batches.  The first batch, I took off the towel and the plastic, and the loaves fell to the top of the pan.  The next time I made these, the loaves raised to the proper height and the proper test by pressing down with the finger, so I placed them in the oven, (without preheating) and started them cooking.  When the time was up, I opened the oven, and the bread had fallen to the top of the pan.  This does not hurt the quality of the bread, but it is bugging me that I can't make a good looking loaf of bread like I use to.

Any help and or ideas would be appreaciated.