There could be various reasons for that, from a badly-behaved app to a hardware fault or corruption in the operating system. The best place to start would be a factory reset (don't forget to back up your content first) and then try it for a few days with no apps added. If it's stable, add your apps one at a time and see if the problem recurs. If it does, you''ve found the culprit. If it doesn't, it should be fixed.
If that doesn't work, then it sounds as though it'll have to go off for repair. This will include a factory reset anyway, so you haven't lost anything by giving it a try first.
