It might be an issue with the power button.
In my case it was the power button, which tends to rattle and its not fit enough.
As I lost the copy of bill, i can't send the device to LG care.
So today morning I tried pressing the power button and as soon as the google logo appears I started to tap fast at the back cover with my finger right behind the power button.
SURPRISE! the device booted succesfully and was using for about couple of minutes.
Again it started to boot loop.
did the same procedure to boot. now my device is running for more than three hours and I don't know when it would again go to bootloop.
Its just the problem with my power button and I will soon get the bill copy from my retailer so that i will send the device for repair/replacement.
Meanwhile I've locked my device and set the tamper flag to false.
EDIT:- Now my device is back to normal. Thanks for your valuable responses. Its just the power button failure.
thanks a lot