iPod touch non-responsive?

I have a 32GB iPod Touch v5 with iOS 6.1.3 with 18GB free. Lately, it suddenly becomes non-reponsive. By that, I mean sometimes I need to tap on an icon many times before it will respond to open an app. Sometimes, I need to turn the iPod off and on again before an app will open. Sometimes, when I read an iBook, I can't turn a page by swiping. Very frustrating.

I just restored my iPod Touch and the problem persists. What else can I do?

Thanks for your help in advance. Tom