Help... blowing hot air through vents

I need the help of this forum troubleshooting this problem. Any idea is welcome.
On my 1991 300E, I noticed warm air was coming out of the air vents when the fan is on even when the temp dial is all the way to the coldest. I did a search on the forum and everything is pointed to the mono valve as the culprit. I bought the whole unit(this model has a sealed unit and does not have the repair kit). I installed the new mono valve and was very confident it will take care of the problem.
Road tested the car and it seems its okay until the following day when I noticed hot air was flowing through the vents again. I observed that hot air comes out of the vents only when I start the car after the engine was already hot from a previous trip. Hot air blows through the vents for about 2-5 minutes then the temperature drops to the outside temp. This happens every time I start the car with the engine at operating temp. What is the possible cause of this?? Anybody out there encountered this problem in the past? Is it posssible that the new mono valve is defective or there are other areas I need to check. How does the mono valve work and how much current is supplied to it.

This what I think is happening. When the car is parked with the engine "hot", some of the coolant is escaping to the heater core through a valve somewhere. This valve however gets shut completely after you start the car. The hot coolant in the heater core causes it to blow hot air for a short period until the outside air temp cools the heater core and by then everything is normal. Are there other valves in the system other the mono valve. Please advise. Thanks!!!!