Nokia 7210 Supernova — defective product

i purchased the above mobile phone on 19.7.09 and have to state that after a month or so it started malfunctioning i visited service centre at dilshad garden , delhi -95 on 2.9.09, than 12.10.2009,than 31.10.2009 , than 6.01.10 and 17.01.10 for rectifying the problem of screen disapearring. but inspite of visiting service centres so many times problem still exists with the mobile. The above set is under warranty at present . we are now fed up . We want product to be exchanged .

please reply satisfactorily as soon as possible