climate control torture

During the 3 years that I've owned my 82 300D, the heater has never really worked right. Last summer I replaced the climate control unit. The AC system was also repaired last summer, including a new compressor.

I drove from Manhattan to northern Vermont last Wednesday, and as usual there was minimal heat. The air coming out of the dash and floor vents was cool but not cold... I think the interior temperature of the car was around 55 degrees.

The Mercedes dealer in Burlington (service dept. recommended by a friend) replaced the mono water valve diaphram, which was torn. That seemed to fix the problem... the temperature in the car was nice and warm as I drove away from the garage. But the next day, as I drove back to New York, the air coming out of the dash and floor vents would change from hot to cold. Each cycle of hot-cold-hot would take about 15 minutes, and repeated itself for the entire trip, about 6.5 hours.

Any ideas?

Thanks in advance. R