1984 300CD Heat question

[jaybutter]

I have a 84 300CD and I am struggling to repair the heat. The problem is that when traveling over 50 MPH there is no heat. At less than 50 MPH the heat is fine. I have rebuilt the mono valve and replaced the aux water pump. The valve seems to operate normally. I can hear it pulsing, it shuts off water flow when in A/c mode and controls the air temp very well at less that 50 MPH. The aux water pump seems normal too. I can hear it running when the engine is off and the system is calling for heat and I can hear the coolant recirculating and I can hear the mono valve pulsing. I just not sure what to check next, any ideas? Again, the air at the outlets gets cold very quickly right at 50 MPH.