Nissan Repair/1987 Nissan pathfinder
When I start my 1987 Pathfinder (carborator) the idle fluxuates up and down on park. Once I'm on Drive you can't feel the fluxuation until youstop on Drive.
Is your "check engine" light on?
If so, then you will have to get the code from an OBD reader and let me know what the number is. Autozone does it for free, I think.
If there is no check engine light on, then the first thing I would do is to clean the throttle body with an approved cleaner compatible to the catalytic converter system of your vehicle. A competent mechanic should be utilized.
Hope this helps -