your fault can be as simple as bad battery terminals. i have experienced same issue in my own car, replaced with bigger battery terminals.
i had self issue in my Baleno, so on recommendation of some people i installed 'diesel wala self' which is considered very powerful - from Liaquatabad market. near star gate, there are dozens of part dealers/electricians - you can safely get your self serviced too or changed too - but before do focus on battery terminals first.