Just a few things ive known and come accross, may help
........... may not
There were issues with these, normally replacing the Lambda Sensor corrected this. Some of these had what was called an A/R ratio sensor which is in fact a Wideband Lambda Sensor on these. How many wires does your Lamdda Sensor have, what values do you get from all pins.
From Memory around 2.5 volts on some Lamda Sensors = 14.7:1 where staggerd to 0.5V = 10.29:1 and upto 4.5V = 22.05:1. Hence the constant switch as the engine ECU cant maintain perfect operating conditions due to running parameters.
There were 'known' problems with these vehicles which had the Renault Running Gear in as the harness's used to get damaged, an ohms check on the wires from Sensor to ECU would verify this. Have you checked the power and grounds to the ECU, an incorrect voltage doesnt always mean a faulty ECU, it could be due to poor power supply or grounding and the ECU trying to power systems respectively, ive had this a lot on certain fords and vauxhalls and on a few Renaults but normally you get performance related issues too and they tend to have done a few miles.
Normally, you can check this at the DLC as the Power is usually picked up from the ECU and you have a ground from chassis and ECU, TIP: with the Can Test Box you can scope the signals checking for any irregularities.
I have seen issues with some of the ECU connectors with connectivity on the terminals, im not saying it is that but always worth a look before condemming the ECU, a dodgy ECT connection for example. Sometimes even on a newish car its not unknown for a terminal to lose contact or for the terminal plating to wear especially if its not been teminated into the connector properly.
NOTE [Found on Internet]: The American Version under went a recall to replace the heat shield as well as the Lambda Sensor as the sensor was overheating and causing it to breakdown internally. The fact you have replaced the sensor kind of eliminates this though.
What year is this vehicle, what engine is it, have you spoken to a Nissan Master Tech, usually they are quite good over the phone, could be simply a ECU recalibration or something like that but if its fairly common he may give you an idea and of course you can then do the work yourself.
I cant remember the key differences between the K11 and K12 other than facelift/model upgrade.
Hopefully someone else on here may deal with these day in day out so can pin point it a bit better for you as its been a while since ive come accross one of these.