manjoo, i tried to search and see if this was a common problem in Storm
The only thing I found is this thread:
BlackBerry Storm: Blackberry Storm 2 (9550) Random Restarts/Reboots? « Blackberry, iPad and iPod Touch
but i remember you mentioned that you see this random reboot only on 3G
I can suggest that you upgrade the OS and see if it helps - alternatively try shifting to Airtel and see how Airtel 3G works on your phone !