Sony Ericsson K790i — appeal towards problem and help

I had purchase Sony Ericsson K790i from Hot spot telephone shoppee at sadashiv peth , Pune on Dated: 24.oct.08 .from December I'm starts facing problem with my handset.Sometimes it's keypads stop working or sometime joystick & sometime Both were not Working.I had visited Service Centre Tilak road, Pune several times.On last visit they took One Month to repair the set & they said that they had replace the Motherboard of the set.Now again I'm facing the same of problem. i gave them again , now also they are taking nearly one and half month. I'm very tired from of situation.sir, it is warranty period . till now they have took 3 months of my warranty period
So,Please arrange to replace my handset otherwise I'll go for other Option(Consumer Court),If my problem is not resolved as earlier.