MObile Nokia 5800
MObile not been repaired after subbmitting it for service 5 times

I bought my mobile on 3rd oct & it is still in warranty period & since then i have submmited it 5 times for repairing purpose (phone automatically gets restart & hangs up) but ebvery time i have been returned teh same hand set without any repair. These guys say that it has under gone repair & will work properly but till today it is not today i again brought my mobile back but still it has restarted twice within 1 hour.