Do you keep getting the 'iTunes has detected a device in recovery mode' message? If so - and this comes from a senior advisor ion Apple Support - do NOT click okay. Just ignore it till it stops downloading and processing the file and the progress bar goes away, so you just see the

logo. Only then should you click OK and then Restore.
The downside of this is, if there are other errors, you won't see them until you do acknowledge that message.
I spent 40 hours trying to restore my phone after the initial download (8 or 8.0.1, not sure) downloaded but failed to install and dumped my phone into recovery mode. So I have been through every possible fix in the book. Apart from not clicking that OK button, I was told to try downloading using another computer and/or network - which i didn't have, but I did take my laptap and sit it right next to the wifi router. Eventually I ended up with 8.0.2.
Other things to try - using a different USB port, different cable, and in some case, though usually not during the download stage, you may need to shut down iTunes and restart in Safe Mode which will disable any third party plugins; just press and hold CTRL-SHIFT and click on the iTunes desktop icon.
I hope some of this is useful for you.