As Mark said, next time move the shifter to N from P.
If it does not start, then hold the key in the start position as you move the shifter [ wiggle] in N.
That will tell you if it is NSS switch or Shifter bushing wear...BOTH vey common on that chassis and both same diagnosis. If the shifter bushing wears , the shifter can not go all the way into P position b/c the shifter gate stops the shifter foward movement, whereas in N you have the wiggle room on both sides of N...