I don't know much about Nissan ECUs, but most ECUs have long- and short-term fuel trims, and some other stored parameters. If the problem you're having is getting progressively worse, the ECU may be trying to override something on the engine, such as a bad MAF or a fuel system change, etc. This should be easy to figure out by simply resetting the ECU. If that is the issue, you should see everything go back to the way it was. More likely though, you probably have a sensor failing. Grab the FSM and see if there are testing and troubleshooting procedures for that and the other critical sensors, like intake air temp (there is usually an IAT sensor on turbo cars with a MAF) and throttle position.