I hope it is not a hardware problem the further reason being that reboot fixes the problem. I feel that S4 was a more solid product and the UI much better. In S5 UI many feature are missing. eg. no lock screen widgets, no control over GPS, the text on the notification bar is too light thus rendering it difficult to read., some apps are crashing on S5 whereas they worked perfectly on S4!
Hope Samsung listen to users and fix the issues.