Why do I have to keep restoring the software in my iPhone?

I have an iPhone 3. Last week I lost my service so I took it to the Apple store; they wiped out the operating system then restored it which fixed the problem. Yesterday I lost service again and restored the operating system myself. Within an hour I lost service again! I restored it yet again and (fingers crossed) it is still working. Any ideas why this keeps happening and how I can stop it?