I upgraded my baseband to 6.15 on 4.2.1. I then tried downgrading to 3.1.2 and using blackra1n to get past error 1500 with the recovery screen. It worked and my baseband was 6.15 on version 3.1.2. I tried using Cydia to install ultrasn0w but it keeps crashing so I tried restoring back to a custom 4.2.1 ipsw. I used redsn0w to put the phone into DFU mode and then used iTunes to restore to the custom 4.2.1 ipsw. It went back to the recovery screen after the restore was done. I tried jailbreaking it with redsn0w to get past the recovery screen but it didn't work. What's the proper way to restore to custom ipsw?