Yosemite Wifi drops often, asked for keychain password

Very recently my macbook air 13" mid-2013 (Yosemite 10.10.5) has been dropping the wifi connection several times a day, after many months of perfect operation and nothing having been changed.

It's always easy to fix: I go to Network Diagnostics and I will see that the Internet and Server lights are yellow or red instead of green, sometimes also red. Sometimes ISP too. I click on continue and select my wifi connection-name, and then am told that "Network Diagnostics wants to Access key "my-connection" in your keychain" , which I allow.

A few seconds later the connection is restored, and all green lights show., and all is perfect, with very good speed.

Sometimes I also see the additional message "Network Change Detected. Your network configuration has changed. Click OK to proceed to the next step"

So it seems to be that the password or login is being re-requested in order to permit the wifi connection, but why should this be and how can I correct the situation?

Additionally to macbook air, I also have an Imac nearby in the same network (but which is asleep very often), and network consists of a Virgin modem/hub, Airport extreme router (but in Bridge mode) and airport express.

An important point is that Set-up was recently perfected with help from this Support Site, and has been working perfectly since, up till recently. Before that was done, I was also experiencing Dropouts (cured by the improved set-up) but these were not able to be solved by allowing access to the Keychain, as described above. (Needed re-starts etc).

Help is much appreciated.