Hey there,
to be honest there are atleast few posible reasons for such problem.
1- Battery problem: it may be problem with connector or battery it self.
2- Soft
3- display- it rarely happends that display cause logo loop so i assume it also could be reason for restarts.
4- Moather board problem.
You could try to open it again but if your not very expierienced with it, i would advice to leave it for some workshop for diagntostics, they are often done for free.