Make an encrypted backup on your computer in iTunes, restore your iPhone and set it up as a new device. Install all the apps you are using back on it and see if it freezes again. If not, restore from the backup in iTunes. If it freezes again, then you know the device is not the problem but your setup is. A clean install will fix this.
Also, please tell us when exactly does it freeze? After doing what tasks or in which app?
Please note that incidental freezes are not that big of a deal and doesn't necessarily have to mean there is something wrong at all.