are you closing unused apps properly in the background? also make sure you're not opening too many browser pages and clearing your browser cache and history now and then... you need to go into your browser settings to do that now and then. Also make sure its on the latest software / firmware... you got to go into the phone settings to check them details out.
if your issue keeps happening even after a hard reset you may need to get it sent to Samsung under a warranty repair or through your network.
