Just a comment...I have a 420 SEL and had a very similar problem...after several trips to the MB service depts - I finally took the climate control out and found the potentiometer had all tree solder joints loose. I soldered these and now works like new. I had hot/cold blasts; erratic fan speeds; very irregular temperatures.

Not sure this helps but the disgnosis was correctly provided on this website by some of the kind people posting directions and help!!!

Good luck!!