I bought a used iphone, and it started to randomly freeze, its not that bad though because to unfreeze it, all i have to do is lock it and unlock it. This problem stopped for a while, but started back again. What is causing this? What can i do to stop it? It doesnt bother me alot, but sometimes when someone is calling me i cant slide to answer or press answer because its frozen. sometimes i tap it hard and it works, or i tap the back hard. Please help me 
Thanks
Thanks