Undoing a tethered jailbreak
Total newbie here-- I have a "tethered jailbreak" via Blackra1n on a late model 3GS (running 3.1.2). I did the jailbreak hoping that my model might not get the tethered jailbreak problem, but sure enough, after each reboot, the phone hangs with the USB plug and the itunes icon. Of course, I can reboot while tethered with BlackRa1n running, but that's really not ideal.
At this point, I'd like to revert to stock firmware. I found an old IPSW of 3.1.2 online (not mine), but iTunes won't authenticate it to put onto my device. Meanwhile, I used pwnage tool to modify that ipsw for a JB build, but I can't seem to get iTunes to restore that onto my iPhone. Looks like iTunes forces the iPhone to reboot in preparation for loading the modified ipsw, at which point the tethered jailbreak kicks in and the iPhone hangs on reboot.
Is this the expected behavior? Would the inability to reboot make it impossible to fix the untethered jailbreak, even if I tried updating to 3.1.3?