I was hoping that one of the ZN owners would chirp in here but seeings how they haven't I'll give you a couple of ideas.
The ZN runs a closed loop fuel injection system. It monitors engine temp, air temp, throttle position and intake pressure to pick a fuel map to follow. On most automobiles, the injection system is an open loop, meaning that the computer monitors all of the above plus exhaust O2 and the computer alters the fuel map accordingly. So when the ZN has a problem and it's not related to a faulty input signal, the computer cannot compensate.
In an emgine running a carburetor, when the fuel ratio is running lean, it will show up at idle with a fluctuating idle similar to what you're describing. So back to the ZN, you're not getting any fault codes and yet you have an issue. You need to look at the fuel pressure regulator and check if it's in spec. Plugged injectors will also cause a problem since the computer is pulsing a value of "X" at idle based on the program but the injectors aren't putting out the right amount of fuel based on a pulse of "X". also if the throttle position sensor is not properly adjusted the computer is not being told the right information to determine what signal to pulse the injectors.
For the record, I haven't worked on the ZN injection system but have vast knowledge on multiple injection systems and can pretty much work my way through most of them based on components of the systems.
Hope these thoughts provoke some testing.
