Here's a couple things to check: Sometimes the injectors lose contact with the electrical connectors because they get corroded or loose(why, I don't know, it really sucks) and this could be causing the problem. With most the Z's I've dealt with, you can actually tell which injectors are "missing" by putting your fingers on the injector itself; you'll feel it "ticking" away. You can also hear it if you've got an automotive stethoscope. Otherwise, remove one plug wire at a time (loosen them with the car off so you're not struggling with them while it's running); each wire you remove that is "hitting" will cause a notable difference in the idle. If you remove a wire and there is no difference, or much less difference than with the others, check that injector's connection and make sure it's clean and tight. Be careful with the little wire bail that holds them on, don't lose it. Also, the plastic connectors are often brittle, so treat them like gold. This is the same method you would use to locate a bad plug... so check the plug on that same cylinder. I've not had spark plug problems with any of my Z's, but that doesn't mean your's doesn't!! If you clean/tighten the injector connector, and clean/replace the plug and you've still got a weak hole, you might also check the distributor cap and the wire that leads to that cylinder. These are the basic things I check whenever my cars start to sputter. Hopefully you'll find your miss!!