Restoring 3G on 3.1.3

[Diver1]

My phone keeps giving me problems and I want to restore and then rejailbreak and preserve my baseband for a unlock. Where do I start? iPhone 3G on 3.1.3 and 5.12.07

[z3r01]

get 3.1.3 ipsw, hold shift (option) and hit restore, select 3.1.3 ipsw, restore , then visit jailbreakme.com and enjoy

[Diver1]

It says could not be restored because the firmware file is not compatible.