Why my ipod touch 4g freezes constantly?

I have dropped my ipod and shattered it before. When I dropped it, I didn't want to have to pay for a whole new one...instead I mailed it to mission repair to replace the screen. When mission repair had it I think that they maybe jiggled a part loose in it. Right after I got it back from them, I started to notice that sometimes on the unlock screen the screen will freeze and not react to me touching it. Sometimes it works great but then it freezes up again!!! Occasionaly it it will freeze for a whole week!! HELP