next time it does it I would pull the connector off of the NSS switch on the side of the trans & see if the middle pin has continuity to the nearby trans metal case. If it does not then as said I would undo the linkage from the lever & with the lever firmly in the park & neutral detents see if it now has continuity. if in the first case the middle pin has continuity then I would check the electrical connectors starting with the NSS plug in connector & all the way up to the starter relay for a poor connection. most all wiring continuity problems are at the terminals/connections.