Hi binnyjeshan,
I have experienced with almost the same issue in my l720.
Case 1: (Day 10)
I tried to open the facebook app. It was taking more time than usual. After a while the phone got restarted.
Case 2: (Day 31 Today)
I was listening to music and wanted to change the song so I tried to unlock the screen but nothing happened. The screen is black but the current song was still running. Volume up down was also not working. I waited for the song to finish to listen the next song but after completing the song the next song was not started.
After reset it shows a big Exclamation (!) sing on the screen and restarted it after some time and this time it’s quite laggy and shows no network coverage and it restart again automatically. After that it’s working fine.
In all my experience I have seen the Network bar shows no coverage when it getting restart again.
Do I need to take my phone to Nokia Care? Please advise.