Cut 3G connection after updating to 4.3.1 iPhone

Recently, i have updated my iPhone4 to 4.3.1. Afterward, my 3G connection was died. A very weak signal or no service were appeared. As a result, I turned off 3G connection. Now, at least, I can receive calls and texts in 2G mode. My iPhone4 works well with Wifi and 2G now.
The following are some information for my using:
I am using 3 as a carrier in Hong Kong
Model: MC603ZP
Serial No: 82037FQSA4S
It was bought in Sep 2010.
Preinstall: iOS 4.1. It worked fine with 4.1, 4.2.1.
I have tried to restore for more than 10 times. The problem still exist. I have googled it. Similar cases were found in iPad discussion. (http://discussions.apple.com/thread.jspa?threadID=2776578&start=0&tstart=0)
It seems that it is a software problem. I wonder should i take my iPhone to Customer Service and change for a renewed one. Hereby, I want to ask do anyone else experiencing similar in using iPhone.
iPhone 4 iOS 4