If we could get the stock 3.1.3 or earlier firmware installed on your phone it would be possible... Unfortunately, there's no way of doing that currently.
In order to install 3.1.3 you need to have the SHSH blobs for that FW saved via Cydia. Basically, after a certain point Apple's servers stop authenticating old firmware. Saurik has a workaround where you trick iTunes into checking with his server instead of Apple's, but in order for this trick to work, your phone had to have been jailbroken on that version firmware and saved the "SHSH blobs" (specific to your phone) to his server using Cydia. Since your phone has never been jailbroken, those SHSH blobs were never saved for 3.1.3, meaning that iTunes won't let you restore to that firmware no matter what.
Major bummer... But hey, just because a few people tell you it can't be done, keep looking. 😉 I had a situation that I thought was hopeless until just recently.
