have a 1998 Model "Kia Mentor" Electrical Problem !?

In the last two weeks, The lights begin to fade, radio stops,
shortly after this happens the Motor Stops.
The car covers 260km four days a week, all at night.
Try to re start the battery sceems to be flat, however
a Jump Start, will have it running to continue the Journey.
It may go a few days OK, but then the same problem ??
I hope you can help, thank You.

Peter Kelly from in land Australia.
March 2007