The fact that your Infinity G20 is taking longer than a few seconds to return to normal idle indicates either the ECU (engine control unit/computer) is not commanding the fuel injectors to cut fuel upon release of the throttle, the injectors are sticking open, the throttle position sensor is not sensing the gas pedal has been released, or the idle air control (IAC) valve is malfunctioning.
We'd recommend first starting with diagnosing the idle air control valve. The idle air control valve is prone to collecting dirt and oil, reducing its ability to open and close quickly.
Your Infiniti G20's idle RPM will change over time due to dirt collecting in the IAC. The ECU will try to compensate as much as possible. The ECU controls a solenoid inside the IAC which allows air to enter the intake manifold in order to control idle speed.
When your Infiniti's alternator load is high the ECU might command the IAC duty-cycle to 20%, when the engine load is high and speed is low you the IAC duty cycle might be as high as 50%. When at a high RPM and getting off the throttle the IAC duty cycle will reduce from 70% to 50% to 30% and so on until idle RPM is reached. With a sticking IAC getting to idle RPM will take longer.
