I have a 96 sentra 90000 mi The car will occasionally skip at highway speed under load. Also at random times the engine will just quit and when I try to start it, will turn over but not fire, after some time the car will start and run fine. I have not been able to isolate it to fuel or spark.
Thanks

Well, do these things in order if the problem isn't fixed:

1. replace your spark plugs with NGKs
2. Get a tune-up(plug wires, distributor and fuel filter)
3. You have a faulty fuel injector. Find it and replace.