iPhone 4 goes dark gray, almost black

Hey,

So about 6 months ago i dropped my iphone on tiles and ever since then it goes to a really, really dark gray screen when the iphone gets relatively hot. When i use apps and games that are a bit heavier it can happen within 1 minute of opening the app or game. The way to temporary fix it, is to lock the phone and unlock it with the sleep button, but sometimes i have to do it 20+ times until it comes back to normal which really frustrates me. I've tried the holding sleep+home buttons and i've tried restoring. Is there anything else i can do(not hardware related). If not, what hardware would possibly be the problem in this case?

If it helps, i have a white iPhone 4 32gb running 6.1.2

Thanks!