I would bet its a bad coil pack. You need to get your CEL codes read at Autozone a find out what cylinder it is. Then you can swap the coil pack from that cylinder that is misfiring, to another. If the misfire follows itself to the cylider where you swapped the coilpack, then the coilpack is bad. I hear they are expensive too.
On a lighter note. My buddy in his 2000 Maxima had the same problem. Random misfire and it is was intermitent. It was cylinder #5 front bank, middle one. He had almost 100K miles on his original plugs. He changed the plugs properly and the problem is gone. He has coil packs too, and they are real expensive on his Nissan too.
2000 Millenia 2.5L
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