Why does my iPhone lose service when I get home after work?

I work about 50 miles from where I live. My iPhone is un-jailbroken and is locked. For iPhone n00bs, it’s just a normal iPhone 3G you’d get from the Apple store or AT&T.

It’s like clock work. I get off of work with 3G coverage. I drive home and get home to find, no coverage until I restart the iPhone. I have service once it’s been restarted, but whenever I arrive home from work in the morning, there’s no service. I’ve even tried leaving it that way to see if it comes back later, but it doesn’t.

Any ideas?