Okay, this is what I did. I have been trying to figure this out for days. I have been driving for a week or two in 0 - 20 degree weather every morning with my boys and they hate me!

I put the old monovalve in and looked around the whole vehicle after talking with an older import mechanic who is a family friend with a 56 merc. He said the one thing he remembered about 300sd's was that vacuum was everything. I searched all over the engine area to make sure that everything was connected in the system and lo and behold right next to the fuel injectors was a tube out of place, which was causing the my engine to sound like someone was on and off the gas pedal. I had be perplexed about why I had an unsteady idle as of late and that seems to have been the culprit. With the old monovalve in I got up to about 90 degrees Celsius and then I heard a click and all of a sudden I got heat at full force. That had certainly never happened before. So now I am afraid to take the old one out and put the new one back in again.