Why does my Samsung Galaxy s4 re start itself and then freeze?

[AC Question]

I recently took my Samsung Galaxy S4 to a shop to have it unlocked to allow me to use another network. However, since then it just restarts itself seemingly at random and then freezes when I unlock the screen for the first time but never the second. I took it back to the shop and they have no idea how to resolve it. Please help! I have done a cache partician wipe as advised but it didn't make any difference.

 

[beagle82]

Is your phone by any chance rooted?

 

[belodion]

Are you using an SD card? It may be bad.
Also, try using another SIM.

Edit, posted same moment as above.

Posted via the Android Central App

 

[B. Diddy]

Welcome to Android Central! Another thing to try is Safe Mode, which temporarily disables all 3rd party apps. http://support.t-mobile.com/docs/DOC-5744

If the problem disappears in Safe Mode, then something you installed is causing the problem. You may have to uninstall apps one by one until the problem disappears. If the problem persists in Safe Mode, then it's more likely to be a hardware or firmware problem.