My home Wi-Fi network setup by Motorola Surfboard Modem (SB 6141) and Airport Extreme (new model) works all right all this time until today. Airport Utility on my iMac shows that "Wi-Fi: on" at one moment and "Wi-Fi: looking for Network" at the next moment. The on-off connection continues. But connection on my iPad Air is stable. I have restarted modem and Airport Extreme, and also restarted and shut down iMac. the unstable connection remains. Any suggestions? My iMac is running OS 10.10.5.