Signal problems with Cingular and 6670

I have a really terrible problem with Cingular... I have a Nokia 6670, bought in Viet Nam, and it is an unlocked one. I brought it to US and sign up a Goplan with Cingular. Be4 the Katrina hurricane, everything worked very well. During the hurricane, i accidentally locked the SIM card, so after that i went to Cingular store and bought a new SIM card on September 6, and everything worked back even though sometimes i had to restarted my phone to get the signal. UNtil Sept 14, my phone didn't get any signal although i tried restarted the phone hundreds of times. So i went to Cingular store again and asked for their advice. They put my SIM card to their phone and it worked, so they told me it was because of my phone. I went home and borrowed my friend's SIM card and put in my cellphone, guess what my phone got full bar of signal, and my friend's phone got full signal with my SIM card too. Once again, i went to Cingular store and told them the problem, they gave me a new SIM card, and tell my to wait 30 minutes for the service recognized the new SIM card. However, after 30 minutes, my phone still didn't get any signal with the new SIM card, and did the same thing as the old SIm card. After a while, with the new SIM card my phone got 1 bar of signal for 1 minutes, and i made a call to my friend's phone. BUt after that, still no signal until now. What should i do now? Can anybody give a advice?