1990 Jeep Cherokee Having problems starting up

Engine Mechanical problem
1990 Jeep Cherokee 6 cyl Four Wheel Drive Automatic

I am having problems starting my Jeep. Just had the altinernator tested and replaced. Will turn over and will hear a "hiccup" like it is wanting to start, but doesn't. Will usually start up when hooked up to another car to get a jump. Can sometimes get it to start on its own if I am standing outside the car reaching in to start it. From my experience, it won't start if I sit in the car to start it. Any suggestions? My father is a mechanic and he is at a loss as to what it could be.