I would simply do a complete tune up…which is not that involved on a 98’ Pathfinder. Change the spark plugs, distributor cap and rotor and even the wires with that many miles on the clock. Change the engine oil (w oil filter of course) and the air filter also.
Pay close attention to the air filter box and all of its connections…like the large black rubber hoses that connect the filter to the intake…look for any cracks or leaks. None of these black hoses or tubes can have any cracks or anything causing air leaks. Remove the MAF sensor and spray it off with brake clean spray or starting fluid…or aerosol MAF cleaner (the others will suffice just fine however). This will clear off any kind of residue or dust or worse from the MAF element so that it can measure Air more accurately (which is very important for fueling).
Without a full and proper tune up… there is no reason to look elsewhere or have anyone here suggest what to do because no one including you know when the last full tune up had been completed…and its vital to how the engine runs. Once you do everything outlined here we can then move on to engine ignition timing and the Air Idle Control Valve if need be.
But you need to do the full tune up first just so you and we know the state of “tune” and maintenance at this point…it may be the only thing you need. We can move on from there if no change was made but I would be a bit shocked to hear that no engine running improvements were made by the tune up.