harrasment in using mobile phone of nokia.repeated problem

Location/place: kolkata

Name of company/service: nokia

i purchased a phone x7-00about 10 months ago.from day 1 there was a problem in phone.it was getting restarted by its own at the time of calling anding my phone.thw=ere was also a problem that phonewas getting stuck in one screen and there was no responce in touch.after repeated visit to nokia care still the problem persisted.they have replaced my phone twice but still the same problem is persisting.they also told me to change my sim and memory cad and i hv done that also and once nokia care formatted my memory card itself but still the problem persist i live in kolkata. i am very fedup wit my phone.pls help