I have a strange problem with my iMac (mid. 2011 21,5) - I can't connect to the network using Ethernet because OS X tells me that it's disconnected when it's plugged in. Sometimes it can see the connection, but the Internet won't work.
Now, I also have this problem with Wi-Fi, it connects to a network but after random time Internet connection stops working. All other computers and phones are working with Ethernet and Wi-Fi; only the iMac isn't.
I tried to do all tests and resets but nothing, today I formatted it returning from Yosemite to Snow Leopard but the problem is still there. Any solution?