Downgrading from 4.2.1 to 3.1.3 I have an iPhone 3g and I'm trying to downgrade it from 4.2.1 to 3.1.3.

I downgraded it successfully and use redsn0w to boot it.  It is factory unlocked by itunes, but it seems like it's not getting any signal though.  Is it because that I have a baseband of 06.15.00?  But how do I downgrade it if that's the issue?

Thanks guys!
I used Cydia to fix the no signal issue with my phone but i dont know if its possible without jailbreaking the phone.