My key pad buttons aren't working correctly. Any ideas why?

I replaced my Iphone 4 last September (shattered screen), and my buttons aren't working correctly now. it's out of warrenty. Sometimes It won't allow me to enter my passcode in, other times, I will be trying to type a message, and it won't let me use certain buttons, instead "predicting" other words. Last week, with my hands off the screen, an entrire message in random letters was typed without me touching the screen. I keep shutting off my phone and restarting it, But it is happening several times a week now, and I don't know what to do!