So here's the thing?
You NEED an older version of iTunes.
If you have a more current version of iTunes you NEED to uninstall it, AND iCloud, AND apple device helper, AND, etc?
Pretty much go into remove programs from control panel and remove EVERYTHING published by apple?
Once that is done, reboot.
Then install an older version of iTunes, for exact compatibility, use this one
http://www.3kzhushou.com/edu/792.html
Then when that is installed reboot again.
Then run iTunes once.
Then you can try and jaikbreak your phone. That should get you around that 20% issue.
Also of note, for me, my phone did not reboot again at 60%.
However, it did reboot twice at the 20%mark and I thought it had hung again.
Also of note, make sure that before you jailbreak, after you've gone through all this iTunes hassle, make sure you turn off your passcode on your iPhone, make sure you turn off find my iPhone, and for the sake of everything holy in this world, make sure you set autolock to never before you do your first run of cydia!!!!!