1997 Jaguar XJ6 Car won't always start

When we are out and about and return to our car, it sometimes will not start. Sometimes it takes 5-10 minutes of us moving the gear shift around from park to drive and back. All of the lights come on in the car. Sometimes if we remember to turn everything off be fore we leave the car (radio, air/heat, etc.) then the car will most times start right-away. What is causing this to happen? Another thing we've noticed when filling the car with gas it gets to the half-way really fast and then slows down until full. Is the tank really full?
Thanks!
Angela and Kelly Mitchell