regarding my new phone being repaired more than 4 times nd mother board changed twice

My new phone samsung galaxy model gt-15510 bill no 4125374566 which was purchased on 10 march 2011 stopped working after a moth .the mother board of which was changed nd my phone was returned back to me after 1nd a half month .again the same problem appeared after 6 months the mother board again changed .still problem was not rectified , thrice i have to send my phone to the service center.As suggested by samsung customer care they say you can demand a new phone if it has been punched more than 4 times.Lucknow samsung office is saying there is no such policy of changing the phone.
hence i would wish to seek your intervention in this matter and provide me a immediate solution to my vows