I have replaced both the battery and alternator.

I have replaced both the battery and alternator within the last three months. The van started first thing this morning, but now will not start at all on its own. It will start with a jump. When I jumped it this morning and drove it home, after driving for a few minutes, the radio shut off on its own, and then the dash lights and gauges started flickering. There is no corrosion on the battery terminals. When I put the key in, nothing turns on at all. Could it already be the battery or alternator again? The starter maybe? I will do the repair myself, but I would like a little guidance before I go out and spend what little money I have left on parts.