Hi,
Check the terminals whether they are fully tight,
how old is your battary, check the water level in batter, rather get it checked by the battery wala, may be battery is giving up
although i dont believe, but people say that central locking and remote, some times drains the battery, if you have any such thing then get it checked
if the problem is only at starting time, then better check the self as well
these are thing I can think of right now
Regards