Wadmancpu

I have an 03' F-150 Supercab Heritage Edition with the 5.4L. On startup the fan runs full speed for about 5 seconds then drops to low speed for about 3-6 seconds then back to full speed again and repeats. I'm not sure about warmer temps right now. My first thought was a defective clutch so I have a new one ready to put on. I just wanted to see what you guys thought before I changed it. My ac pressures are also low which I attributed to the fan running too fast too often keeping the condenser temps too low. I just vacuumed the system down and charged by weight to get the exact amount of refrigerant in the system so I know its not a charge issue and I don't believe there is any compressor issues. From what I know of these thermal clutches they should never cycle this rapidly. What do you guys think?