This Device isn't Eligible for Requested Build restore error in iTunes
I would take your phone to another computer that works well to restore devices (hopefully that exact device if possible). With so many variables in the restore process, it's nice to be sure it's not a malfunctioning iOS device before reinstalling Mac OS / iTunes.

Once you rule out the device having issues - do run through these two articles (and yes, you may have done most or all the steps). You might want to try forcing your phone into recovery mode. Power it off, plug in USB to power or a computer and hold the home button until it goes into recovery mode. This will erase everything if you restore from recovery mode.


iTunes: Advanced iTunes Store troubleshooting - covers time, firewall, hosts, etc...
iTunes: Specific update-and-restore error messages and advanced troubleshooting - covers restore mode, and other potential issues.