That's a tough one to follow but here's my take -- the fan should not stop while the ACC is on. My rough guess, and it's very rough, is that your fan brushes and/or bearings are worn and it's hard to keep the fan turning slowly and it can't quite pick up again once it gets to truning slowly. Could also be dead resistors in the resistor pack.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD