1978 Other Porsche Models Car needs a jump when ever I try t

Electrical problem
1978 Other Porsche Models 4 cyl Wheel Drive Type unknown Manual 99,999 miles

Every morning I am able to start the car up, and I drive it to city parking garage. After An hour or two I come back and I try to start the car (Manual transmission) with my foot press down on the clutch, turning the key and nothing. I tried giving the car some pumps on the gass pedal which worked before, but the only thing that seeems to work is jump starting the car which fires the car back up. I know that my car dealer put new spark plugs in it and I did an altenator test which I drove it back to the house and let it run for 15 mi utes and none of the lights faded nor the battery warning light came on which is a good thing. I don't think it is the starter because I hear no clicking nose when it starts up or shuts off. What could it be?