If you are trying to downgrade from ios8.3 to ios8.2 as the OP has suggested using the 8.2 IPSW file, you may find that you will need to keep trying. It took me about 90 attempts. The first 89 r sp failed with '3014' 'unknown' errors. After a bit of research (and not a little panic), I found some answers on the apple forums. There seems to be a consensus building that the Apple verification servers are really busy and it therefore takes a fair few attempts before you can get a hit. After all that my iPad 3 and GNS 5870 are now happily remarried and talking to each other once again.