3g stuck in 3.1.3 after downgrade

[jeepville]

I have an iphone 3g that was upgraded to ios4.0 it ran terrible and crashed a lot so I looked up how to downgrade. So I did a shift restore in itunes and put 3.1.3 on it and used recboot to reboot it. So it runs fine now but it wont restore so it lost everything that was on it pics photos etc. It says wont restore 4.0 back to 3.1.3 in itunes.

I have tried upgrading back to 4.0 and even 4.0.1 and now they all fail and its stuck at 3.1.3
I am guessing I still have 4.0 baseband with 3.1.3 firmware which is failing the upgrade check.
This phone has never been jailbroken.
Can anyone help?
I have tried everything I can think of short of going to the genius bar.

things I have tried.
jailbreaking 3.1.3 <- froze on the harddisk pic on the screen
jailbreaking 4.0 <- said successful rebooted phone and nothing changed no cydia still at 3.1.3
hitting restore in itunes says wants to update and restore to 4.0.1 update fails.
hitting shift restore in itunes pointing to 4.0 iphone in dfu mode < restore fails
hitting shift restore in itunes pointing to 4.0.1 iphone in dfu mode < restore fails
hitting shift restore in itunes pointing to 3.1.3 iphone in dfu mode < restore fails