I'm assuming you are "back home"? You may want to turn off cellular roaming. If you have Verizon you could do a *228 and make sure your PRL is up to date. In System Services, under location services, you can turn all of those off with no consequence. Also, you may want to briefly shut off location services and then turn them back on one by one and see if one of them is a "culprit". I would also do a reset by holding down the off button and the home button at the same time until the apple logo appears (about 10 seconds), then release both and let the phone reboot.
Not really sure what it could be, but those are some things I'd try. And then I'd get in touch with someone at a corporate store with your cell provider. They are much more tech savvy than a franchise store.
Let us know if you get it figured out!