I have an iPhone 3G (8GB) that is on 4.2.1 and was jailbroke with greenpois0n along with the iPad baseband update (06.15.00) to allow unlocking with ultrasn0w.

The phone is very slow and bordering unusable and I'm wanting to downgrade to iOS 3.1.3.

Is it possible to downgrade will the different baseband affect it ? also a question of how to do it as it wont accept a legit ipsw only custom one due to having the above baseband?

Any tips tutorial links would be greatly appreciated..