Dear Arham, may i know which model of nokia is it?
This problem has been there for certain nokia mobile sets that have been manufactured and shipped from China.
Do check it up, whats printed on the serial-sticker on the back of the phone (below the battery).

Also its usually caused by the battery which has an unstable voltage power, to date not many has been able to rectify this problem but then its sure to do with the battery!
~u would want to check the battery as well..

*Last resort, do make a report on the shop where u made the purchase from and also I am pretty sure nokia care would help you as u are covered under the warranty period..
~just a little trouble for u calling n walking in to them..
=)

Regards,