Hi Christian. It maybe that one of the relays is playing up or the PCB (Printed Circuit Board) board sometimes the wire I the control box needs swapping over from grey to red, when did you change the fan from 3 speed to 2 speed
So you are saying the aircon brings the rad fan on slow speed for a short while and then the fan switches up to high speed? If so it may be the condensor isn't cooling the r134a sufficiently. If the a/c contents are not cooled enough the pressure rises in the system this then activates the trinary switch to induce extra cooling to the system by moving the fan on to the next speed.