Nokia 5800 — phone switches off automatically but Nokia Care is unable to solve it

Address:

Srinagar, Jammu and Kashmir

Sir,
I purchased nokia 5800 xpressmusic. It worked quite satisfactory for few months but after that it started switching off automatically. I took it to nokia care in Karan Nagar Srinagar three times a month. They all time returned it back to me with OK label on its back. But all time when I reached home problem was there. Nokia care is unable to solve it. I told them if they are unable to solve so please forward the handset and send it to outside or company but they refused to. They think that there is on engineer who could solve it if they could not. Please and please help me. Please tell me what to do now.
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