Update: I re-ran all tests over the week end;
Mono Valve: operates OK as it defaults to full heat when disconnected.
Auxiliary water pump: Operational as it should. Vacuum: I could not replicate the vacuum lost I saw the other day. I must have screwed-up on the first test. Vacuum is normal, engine idle very smooth at 700RPM. Disconnection a major vacuum line make it idle very low and shaky, which is normal.
I spent all day today at my Mercedes Stealership trying to diagnose the problem. 7 hours later at $88.00/hours+shop supply+1qt of coolant++++++=$775.00 to tell me they have checked everything and they could not find the problem. But they acknoledged the problem is real.
Still the same problem: Heater works OK until the car idles for about 1 minute it then switches to cold A/C. While on a/C, idle one minute and it turns the heater on. A tap on the gas pedal reverts the system to where it should be.They tell me they checked the heater controls, tried a new one on, still the same problem. What's next.???