Okay….
You probably have excessive resistance somewhere in the circuit.
I'd bet if you read the cranking voltage at the starter, it will be too low.
I would start by checking all the cables, wires for dirt/corrosion/loose connections etc.
Pay particular attention to your battery cables.
Check the following link, it is a detailed procedure to check your starting and charging circuits; it's better than any quick troubleshooting procedure I could give you:
http://www.aa1car.com/library/voltage_drop_testing.htm
Hope this help