C6
Handset doesnt work properly even after giving it to nokia care thrice

I have got a new handset on 2nd of october. bt just after 3 days it was not working properly. the keys stops working in between. it neither switches off then. i have given it to nokia care thrice. still my problem is not solved. they kept my handset with them for mor than 15 days n still it has the same problem. i request yoy to kindly solve my problem or get the handset exchanged.