Do you mean it randomly restarts by itself?
If so, then there is some problem. Could you try removing your sd card and check if it still happens. Also, SIM cards are sometimes an issue. Rarely, but it happens.
Potentially, some app could give you issues as well.
If it still happens, you could try a factory reset.
You could also take a bug report from developer options to make Nokia aware of the issue.
If all fails and you still get random restarts, you might have a faulty device :(