Nokia 101 auto restarts even after re-flashing firmware

I brought Nokia 101 mobile phone 12 days ago. It is GSM phone with dual sim card facility. It has mp3 player with upto 16 GB micro SD support. My problem is my phone is restarted automatically after few minute. I took phone at Nokia service center and they flash firmware and give it back to me. After 1 day I notice same problem in my phone. I don’t understand what the problem is because after flashing same problem is occurred. I talk to my retailer about this problem and ask him to replace the handset but he refuse to do these. Anyone know what problem is this and how to solve it?

What I know this kind of problem occur mostly because of software issue. But as you mention in Question you already flashed your phone and it is does not solve your problem. So, I think sometimes that kind of problem is occurred because of loose connectivity of sim card. Check your both sim card and make sure both sim cards are fixed properly.

Auto restart of phone is not only software issue but some time there is problem along with hardware also. I think you have to go back to Nokia care center one more time and talk to them about your problem in detail. They will take your phone few days and give it to RND department and they try different solution on your phone. I think sometimes some IC are damage because of overheating or any other issue appear. So to fix that they might replace the pcb and if the problem still persist they will also give you a replacement. They take minimum 8 working days to give the replacement of your phone as a Nokia company policy. Retailer cannot give the replacement because your phone is more than 7 days old.