You can use this website (http://www.felixbruns.de/iPod/firmware/) to download the firmware that you are looking for. Once you have downloaded it save it to your computer desktop. Go to iTunes, press "RESTORE" while holding down the "OPTION" key for Mac or "SHIFT" key for Windows and a window will pop up, locate the file you downloaded and select it, it will start downgrading your phone to 7.1.2. Oh and when you plug your phone to iTunes make sure you switch off your phone and plug it in while holding down the home button, only then you restore! This should work unless Apple has stopped verifying 7.1.2. Nevertheless, give it a try! Make sure to back up before doing ANYTHING. Good luck!
*UPDATE* - Apple is still signing 7.1.2, so it should work. But for how long they will continue signing I am not sure. So do it fast.