Can't make call, text, or access internet.

So here is my problem. For the past couple of weeks I have been unable to text, call or access internet. My service will work for about 10 minutes then cut off again. During this time it says I have service. I've tried resetting network settings, restoring the phone, and even replacing the sim card. The only thing that I have found to work is to turn the 3g service off, now the phone seems to work. I have an iphone 3gs with the newest firmware. Any suggestions would be much appreciated.