No Sim message iPhone 5
About a month or two ago my iPhone started randomly switching over to No Sim. It is annoying but it can be easily fixed with a reset (holding home+lock). What I want to know is if it's a hardware related issue, software, or if I need a new Sim card. It didn't do this under iOS 6 or early versions of iOS 7 and only after I updated to iOS 7.0.4. I'd like to know if I need to go to Apple, AT&T, or reinstall iOS 7.
iPhone 5, iOS 7.0.4