Try resetting network settings, try reinstalling ultrasn0w, try uninstalling any toggles like sbsettings bossprefs for 3g, edge, etc. (that could cause conflicts when one's on and the other off), try 2G with data off, switching airplane mode on/off a few times.
My 3G had antenna problems when I got it off some guy for real cheap, believe it or not restoring firmware and unlocking it made it work.