1985 Volkswagen Jetta CAR WON'T START WHEN HOT

Electrical problem
1985 Volkswagen Jetta 4 cyl Front Wheel Drive Automatic

So my jetta just recently stopped running while hot. The car will start just fine in the morning but after I drive it for about 15 min or more then park it, when I try to start it again nothing. The car doesn't turn over. The battery is new and starter is new. But if I have someone jump the car sometimes it will start up. Im not sure whats wrong with it