I assume you replaced the complete distributor including rotor and cap. If not, are these still originals? If yes, I would start with replacing the rotor and distributor cap. I replaced these along with the spark plug wires around 168K miles and it brought my van back to steady and smooth idle.
If you want to check the spark plug wires? check their resistance with an ohm-meter.
Cyl 1 6.5K ohm max
Cyl 2 10.0K ohm max
Cyl 3 8.5K ohm max
Cyl 4 12.5k ohm max
Cyl 5 8.5k phm max
Cyl 6 11.0k ohm max
If they are over those max values or too close, replace them with a new set of wires. Use prestolite brand. That is one of the original brands used by Nissan on these engines. I bought my current set at http://www.partsgeek.com for about $66.00.