My Iphone 4 messing up

Well, I got my Iphone 4 about a month ago brand new, I had problems to where it would freeze and shut down, so i went to the apple store and got it fixed so thi is my second one, It's the white 8gb model don't know if that matters anyways two days ago my phone completly shut off, fully charged, wouldnt turn back on no matter what put it on the charger didn't turn back on and was just messed up so i kept messing around holding down all the buttons and it finally turned back on. it worked fine but now is starting to loose it's singnal, ever now and then, drops calls, and is very annoying. now this probelm started today it happens sometimes then not others but i will be texting people and then the u,o, and p letters wont get recognized so i have to like slam the button and sometimes it will work but other times it wont so I'm wondering what could be causing this and why it is doing this. also this Iphone is for ATT