Why does my iPhone 4s randomly turn off?

So about last week, I dropped my phone and it went black with blue vertical lines. So I took it to a store and got it fixed. The LCD was broken and they just needed to replace the screen. A few days after and currently, my phone has been randomly shutting off when I'm using it. Sometimes it will even restart, but often it just turns off. It takes forever to turn it back on. When it turns off, it has charge. There is no reason for it to be turning off by itself. I brought it back to the store yesterday and they said my battery was defective, so I got a new battery installed into my phone, but it keeps turning off!! What do I do???