iPhone 5s Replacement Freezing... AGAIN!

I returned my supposedly faulty iPhone 5S to Apple for a replacement as it repeatedly froze on both the screen and buttons, allowing only a manual restart. PAIN! The old phone was restored to new and tested, it still froze therefore Apple diagnosed it as a hardware problem specific to that single phone. I recieved a replacement phone, have had it for two days and it has frozen AGAIN... 6 times on both default and 3rd party apps. Is there any way to fix this without contacting Apple again. I guess this is a software defect, how is this resolved? Any help and suggestions appreciated, thanks!