Jeff 73 smith

I have a 2008 Chevy Silverado replaced resister and fan motor for AC I have digital auto AC controls all settings work with the ac on high the fan will turn on and off at times it doesn't do it all the time seems to do it more at idle then while driving but does do it while driving just not as much put a volt meter at the resister volts go from 13 to 8 when this happens please help me to figure out the fix update I now have replaced the hvac head unit control module in the dash still the same problem tried extra ground to motor still same problem have replaced all fuses relays alternator and battery still same problem any help would be very appreciated