make sure the sim tray is flush with the top of the phone and that the aim card isn't bent Update: sorry sim card isn't bent, I had this problem with a 3G it kept droping 3G signal then turning off then saying connect to I tunes then no service. I took out the sim card and the tray blew down the hole in the phone put sim back in and it has sorted my problem hope this helps..
Answer to: iPhone 3G keeps losing signal/service