iPhone 4s no longer reliably connects to 3G on Sprint

Hi, y'all - about three days ago, my iPhone 4s no longer reliably connects to the Sprint 3G network. My girlfriend's identical iPhone 4s does connect in the exact same physical places, so I'm thinking I've got a faulty 3G radio thingy.

So......what do I do?

Are their any tricks I can try to get it working again?

What are my (no warranty or coverage plan) service options?

Thanks!