The heater in my 380sl seems to only work in town at low speeds. Once the engine warms up, and the speed increases such as highway speeds, the heater blows cold no matter what the setting is. Once you slow down as in stop and go traffic, or if you turn the car off, let it set a few minutes then restart it, the heater blows hot, until you reach high speeds again. I have checked the heater control valve, and it opens and closes. ( I have even disconnected it and the heat still doesn't work right.) I have also checked and replaced the aux. water pump. Any suggestions and help would be appreciated.
Thanks in advance.