Help-Lose signal and only reboot can search it back iPhone 3G

Hi:
These days (after upgrading iOS to 4.3.1, baseband 05.16.02), if my iPhone 3GS is located at a poor signal area (in the elevator of my office building), the indicator will be No Service. But even after I moving to another place or, switch on/off to airplane mode to try to search the carrier again, it doesnt work anymore. Only to reboot the iPhone, the carrier can be searched and signal will be back.
I remember switch to a airplane mode and switch back it should work usually. I dont like to reboot my device several time a day.
Any ideas how to fix it? -- Appreciate for all the reply.
iPhone 3GS iOS 4.3.1 iOS 4