IPhone 5 power button not responsive
Honestly, this sounds like a hardware problem. It wouldn't be software causing that. Chances are it's simply a bad switch. Hopefully you have AppleCare. If so, wait until it gets bad then take it in. They probably would just replace it. Otherwise - purchase AppleCare if you're still within the time. ;-) 

I suppose there is also the possibility something like that could come from something somehow getting in the switch, or damage from a drop. Likely not the case for you, but I think it could happen.

Again, in conclusion, it's probably a bad switch.