I have the same issue. I had been trying various solutions. I spent 20 hours and finally i restore my mac(erase the disk. In 2 days i had the same problem and i understood that this could't go on. I found solution and it was very simple. In Preferences -> Network i change my location from "automatic" to my own. Then you should "off" your wifi at all and "on" it again. It WORKS IT WORKS I erase my mac..... I think that it would help you and save time.