iPhone Help with "USB Communicatin Problem" after IOS Downgrade

I am trying to unlock a friend's iPhone 3g.

She previously installed iOS 4.2.1. I downgraded her iOS to 3.1.3 after downloading the ipsw using iTunes. When it gave me the expected 1015 error, I used TinyUmbrella to exit recovery mode.

The Baseband is set to 05.15.04, so I am planning on setting it to the iPad baseband (06.15.00) using RedSn0w. When I put the iPhone into DFU mode, RedSn0w returns "USB Communication Problem". I tried RedSn0w before performing the iOS downgrade and it communicated with the iPhone just fine. I have done it many times. At this point, I have tried restarting my computer and the devices. Nothing seems to work.

Advice Please!

iTunes 11.0
RedSn0w_mac_0.9.15b1
TinyUmbrella v5.10.15

Thanks dhlizard.

In fact, if possible, I am hoping to downgrade to 5.13.xx. At the moment though, I am stuck because I am unable to get RedSn0w to communicate with my devise. Any insights there would be excellent, then I'll try to tackle the downgrade issue