I'm sure you know that a torn monovalve diaphragm will cause more heat than you want, not less.

I'm also sure that it has already occurred to you that you could have a bad thermostat. I'm not sure how the V8 thermostat is arranged, but I fully expect that it has a bypass as the other engines do. I have had a few thermostats fail open, or in some cases not closed fully. If you haven't replaced the thermostat in a long time, it might not be a bad idea given that it is an inexpensive item.