What would cause a misfire on a 1992 Nissan Maxima with 85000 miles?

In order to have complete combustion you need only a few things.
-Spark
-Fuel
-Air
-Compression
-Correct Timing

Any of these things could cause a misfire.
A common problem with the 1992 Nissan Maxima is the fuel injectors. I would check the resistance on them and check the wiring harness.