Q: An unknown error occurred 1671 - iPad 4 Retina

Greetings soumendra,

Thanks for using the Apple Support Communities!

I understand that you have a frustrating issue where your iPad is restarting itself. It sounds like you have tried some great troubleshooting as well, but when you try to restore you are receiving an alert that an unknown error occurred (1671).

To begin troubleshooting this situation, if you have not completed these steps already, I would suggest reading over and working through the steps suggested in the following article. 

Resolve iOS update and restore errors in iTunes - Apple Support

You also mentioned using DFU mode. Do you mean that you tried to restore using recovery mode with the steps described in the following article? 

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

If you have attempted both sets of steps and the issue continues, I would suggest reading over the next article for information on how to proceed. 

If your iOS device restarts or displays the Apple logo unexpectedly - Apple Support

Have a great day!

Hello again soumendra,

I really hate to hear that your iPad continues to have this error after all of the troubleshooting. Because the issue continues, it may be necessary to follow the instructions from the last link provided previously and contact Apple Support. We will better be able to answer your questions about service as well. 

Make sure that your device is up to date and your apps are up to date. If you still need help after updating, contact Apple Support.

If your iOS device restarts or displays the Apple logo unexpectedly - Apple Support

Best!