I think suginami hit the nail on the head with the lazy O2 sensor. I've seen this happen MANY times. If you want to do a simple test, try this. Monitor the O2 sensor voltage after the engine is at operating temperature. Hold the RPM at about 2,500 for a few seconds. Then, abrubptly floor the accelerator for a split second. When the throttle springs back to the closed position it will activate the "decel shut-off" feature. The O2 sensor voltage should drop exactly as it would during air injection. If the voltage is close to what it is during air injection the O2 sensor(s) are definitely lazy. Good luck