There's been some discussion about this on the forum and there's an FAQ about this.
Basically, some warmth during charging and extensive use is normal. However if the heat is significant (makes it unpleasant to touch) and persists even after you've closed apps and taken it off charge, it might be worth taking it to a care center.
Hope this helps,
i Also have aa same Problem With my phone. its temperature is around 42 'C when I am using music app.
when I am outside and trying to use my camera whole functionalities hangs up. and phone restarted automatically. I also have a problem with a battery I just noticed that it drains faster than regular mobiles. please help me with this problem
Thing is where do you live and what is the ambient temperature?
My house is quite cool today and the idle temp of my Nokia 8 is 25c.
I was outside in direct sun the other day where it was 26c in the shade and my Nokia 8 was 44c idle.