Galaxy S3 heats up while using net or data My phone gets very hot while using internet or data and it starts going slow. Can you tell me if something is physically wrong or give any advice on how to rectify the problem.
There have been several reports of the S3 experiencing heat problems.


  Many users have reported problems with their Galaxy S3 overheating and
  sometimes even freezing or crashing as a result. The S3 is very thin
  with a big screen, a big battery, and a powerful set of internal
  components, so the more you use it, the hotter it will get. This
  generally only rears its head as an issue after prolonged gaming or if
  you do something else that’s graphically intensive. If your Galaxy S3
  is getting very hot and crashing without intensive use, then you might
  have a hardware problem.
  
  Solution: The obvious solution is to take a break for a few minutes
  and let the phone cool down. If you’ve been running something for
  hours, then it will inevitably heat up. If it is heating up on standby
  or with low levels of usage, then you should contact the seller or
  carrier and check if the hardware is faulty. You should be able to get
  a replacement handset if they confirm the problem.


Read more: http://www.digitaltrends.com/mobile/galaxy-s3-problems-what-users-complain-about-the-most/#ixzz3XabgupCo 
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If you use a case for your phone, make sure the case is appropriate for the phone so that any heat can dissipate.  You may also want to try an app like Clean Master that can warn you if the phone is experiencing a heat problem.  Beyond that, you may have a rogue app that could be responsible.