Blower fan suddenly goes from low to high

[jbach36]

Totally out of nowhere, my blower fan goes from the ANNOYINGLY TOO LOW setting, to the high setting, all on its own about 1 minute after starting the car.

(I wish Mercedes would have made the low about 50% more air flow than it gives; the low setting is just too low).

What would cause it go change speeds on its own? I tried it in both the EC (economy with no compressor), and the a/c position, and it does it on both settings. I tried the medium and high settings and those seemed to be fine, the fan speed didn't change when in those modes. I put in a new MB blower fan and regulator probably 5 years ago.

Thanks,

jeff 1991 300d, 180k