Nokia/5800
Switching off mobile at sudden cases

I have a nokia mobile 5800

and i purchased it half an year ago

but i have a problem with my phone

and the problem is that my phone gets automatically swithed off whenever a msg comes or i receive a call or i do any call

my phone have been gone 2-3 times to nokia care

but again i got same respond, means the same problem occurs

then i went to delhi for the problem conclusion, so i gave my phone to delhi nokia care

but again the same problem occured, so what to do

i wished to break up my phone, but i didnt did it so plz give me a solution by which my phone gets correct otherwise u have to refund my money back

or u have to replace it with new once

and do u know i have also wasted 15 hundred more for this problem

and i dont trust nokia any more because at first i thought to buy only and only nokia phone

but now i dont believe it any more

my mobile IMEI no. is 357988/03/645095/0