This is something I had a great deal of experience with lately. If you study the placement of the R/S auxiliary fan motor you will notice that it blocks air flow to the Fan Clutch hub. Since the bimetal strip gets little heated air it does not have the opportunity to heat up thus never really allowing the fan to fully engage.

Solution: On the blue 2-pole switch add a 2K 15 turn potentiometer. Dial the resistance in at 1405 ohms and solder it in parallel to the female contacts.

Let me know how this works for you, I have personally resolved 4 vehicles to date. One of the added benefits to making this modification is the oil pressure will no longer drop below one bar while driving around on very hot days.

Good luck