My Airport continuously and intermittently searches for Wifi networks

I have read numerous queries regarding this same issue. I was informed by more than one that the problem is with Yosemite. As such I have upgraded to OS 10.11 El Capitan, which has made no progress on this problem. The airport is always looking for networks even though it is connected to my DSL modem network, and I have an ethernet - thunderbridge also connected to the same router, and still I cannot stream a small video from Facebook without it stopping to look for a wifi signal numerous times. To download El Capitan I had to turn off all other bandwidth using devices, all extension routers, and blue tooth on my mac, however it still required 31 hours to download the upgraded OS. I have worked with my ISP and there appears to be no problem there. I plugged in a PC laptop which had no wifi problems. Meanwhile my brand new Mac Book Air cannot connect to i-tunes. I have tried numerous other ideas offered off Apple help, which have provided no help. As such, I am asking for assistance with this issue.