Just this past week repaired a 1986 300E that
the compressor would only stay engaged if the motor was running at 2500 or higher....
convert that to road speed if you like. The
RPM sensor reading was 215 ohms and not within MB specs of 590ohms or +/- 10% of that figure, so replaced it and now it is back to normal.
Sounds close to your problem....

Good luck,
TobiasMB
92 300SE
88 300CE
84 190/5.0