I normally associate restarts with smartphones where there is too much going on in the background so it randomly reboots but I got a feeling you're not one of them that leaves multiple windows open and active in the background.
sounds like a drag bud but I would back up what important data you have... hard reset the phone... restore your stuff then see how it goes. if its still happening then its a warranty job. just seen your comment where you said its happening from when you got it... back in August.
dunno if you got it from Samsung or your network but if its still happening I would contact one them... preferably Samsung.