iPhone 5, Sprint, No Wifi unless

I have a new iPhone 5 with sprint service. the 3g network is working fine, not LTE service ever, I have updated and restore many times already, I have the iOS 6.1.2. When I have it connected to my computer using iTunes I have wifi and disconnect it and a few minutes later I no longer have it. when I connect it again it works again. that I the only time i have wifi.

Please someone help, I can reset my phone or restore my phone again with out pulling my hair out....