Blower speed can definately have an effect on this. High blower speed will cause evaporator temperature to be higher than lower blower speeds. This means that on high speed blower there is a lesser pressure difference between high and low side thus less strain on the compressor.
You have one of a few problems.

1. Belt slipping (belt too loose or compressor siezing under full load)

2. Bad RPM sensor in compressor.

3. Bad klima relay.

4. Faulty wiring from rpm sensor to klima relay.

MB doc is exactly correct and you are correct not to doubt his expertise.

Brian

__________________
Mercedes Benz Master Tech
ASE Master Technician
Diagnostic Technician
Owner of "Behind the Star Inc."
Raleigh NC.
www.behindthestar.com
brian@behindthestar.com