Downgrade iPhone 3GS from iOS4 to 3.1.3

Malcolm,

Many thanks for your suggestion. However, my main concern is to get my phone fixed asap with as little disruption as possible. Sending my phone away to be fixed is far from ideal. The simplest way would be to allow me to roll back to a version of the software I was happily using this time last week. I can then wait for Apple to test v4.0 more thoroughly and when it's fixed I will upgrade.

My problem is that Apple have broken my phone and are refusing to allow me to do the most logical fix, which is rolling back to the last working version. Or they offer me a free replacement - but that still involves a 50km drive for me and 2 hours out of my day... not my ideal.

Have you tried a Restore and setting it up as a new phone, not from backup? If that doesn't work, try Recovery mode.
<http://support.apple.com/kb/HT1808>

look .. i just restored my iPhone 3G back to factory defaults using itunes and it become fast like before .. with the new features .. no app crashes .. things are back fine and normal

Incase you still wanna downgrade try this guide .. i didn't try it myself
http://www.addictivetips.com/mobile/downgrade-iphone-3g-3gs-from-ios-4-to-3-1-2/