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I am installing apps/games only from the Google Play Store, not from other 3rd party Android app store.


Then you should be able to systematically work out which app is causing the issue.

Start from a clean slate and slowly install each of your apps. If it works without issue install the next and so forth until you begin having issues. Then look at what you last installed as it likely is the cause.
 
The Facebook app often gave me similar issues on my Note 2 FWIW.
 
Also check what widgets you are utilising. By their very nature they could be keeper my an app or background service running and consuming your ram & battery. Some of the simplest widgets I have found can consume a lot of resources or cause memory holes.
 
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