Sony Xperia Z1 occasionally not responding

[Jek]

Hi, just wanna ask if anyone is encountering occasional not responding on their Z1? I am frequently encountering the System not responding message box. Even if I am not using the phone. When I try to unlock the phone sometimes it just does not respond for a while then just run again after a while. Not sure if this is software issue or hardware? 

Android version: 4.3

Also, I am not encountering this before. Just recently and I have not yet updated my OS since i bought it.

Any info would be really great!

[Thommo]

Have you installed any apps that you may consider unwanted which could be using up system performance thus slowing down your phone to the extent that it gives this message - All I suggest is backing up all info and performing a system repair then monitor your situation - If it continues without installing any apps then I'd consider it to be a hardware fault - If nothing happens and it perform well then install apps one at a time and monitor if only to see if you can track down the app if it is in question causing this issue