Why does my phone touch-screen keep freezing???

[DanL6]

I can barely type a message without the screen freezing; in order to fix i I have to turn the screen off, re-enter the pin (I sometimes have to do this multiple times) and start typing again. This happens so often though it means I spend about a minute writing one message! I've installed cleaning apps to get rid of junk files but it's still happening. Is this a software issue I can easily fix? Or do I need to get it replaced?