The climate control in my '86 300E acted just like that. It would blow hot, then cool, then hot. Eventually it seemed to even out a bit. After reading all these threads on the sampler blower, sensor, etc., I went out to the car to check it out.

First, I held a small strip of paper up to the sensor grill next to the front dome light. The suction of the sampler blower held the paper to the grill. I concluded the sampler blower and its tubing were good.

Next, I looked into the sensor grill. There was lint in the grill. I removed the lint with a toothpick. Using a small flashlight, I could then see the sensor element. It had a white fuzzy appearance. I decided that there might be some lint stuck to it. I took a can of spray cleaner-degreaser used for electronic parts cleaning and sprayed up into the grill. The sensor came out clean and it was a bright blue in color!

The climate control system functioned very well in a short drive test. It seemed to be over its hot - cool - hot cycling. Time will tell but I think the layer of lint on the sensor insulated it from the sampled air.

If you use this type of spray cleaner, be sure it say "safe for plastics" on it.