If the car won't start in Park but does in Neutral, the starter safety switch is going bad. This switch (on the transmission) will only allow the starter to run in Park or Neutral so you don't start the car, with your foot on the gas, in gear -- it will immediately leap forward (or backward).
A bad shift linkage bushing will sometimes cause trouble, too, by not pushing the linkage all the way into the Park position.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!