General :: Pantech Sky LTE A800s Lost Touch Sensitivity Completely

I have a Pantech sky LTE A800s android phone. Recently, I've having this random lost of touch sensitivity!!! After having this problem for a few weeks, My phone suddenly lost its touch sensitivity completely. I had it rooted and it's running on a Vietnamese custom ROM then recently updated it to the latest Jelly-bean with the stock ROM and root-removed. And, this problem only happen in the lock screen. If I've got through the lock screen(If the sensitivity works), everything works fine. Also, all the hard keys work fine throughout this problem.

I've tried to install different ROMs and factory reset several times but nothing worked. I brought it to te store where I bought it the other day and they told me that this can be fixed but it'd cost more than a new one.

Here is the picture of the phone if you're not familiar with it:

(This phone is also running on the new stock ROM)