I have a 3GS that is currently tethered at 4.2.1 BB06.15.01. I procured this 3GS when it was not working, and had a friend bring it back to life, had a 16xx error, and wouldn't come out of DFU mode.
I have tried GP from both mac and windows machine, and it seems to be in a reboot loop, GP runs correctly, then I get GP logo, then reboots to the apple logo, then just continually reboots.
I can use redsn0w to boot tethered and get it back.
I would like the untethered, but am concerned about f'n thing up restoring a pwnagetool ipsw or potentially upgrading to 4.3 when the JB unlock is available.
I have tried GP about 8 times so far with the exact same results.