Not everything has an error code, so did the Nissan tech drive it and feel the problem?
A lot of people will be thinking 'fuelling' - air getting in after maybe a filter change. Maybe a bad injector. How long have you had the problem? Has anything been changed just prior to the problem? The engine is electronically backed-off for autobox changes, so maybe it is when fuel is re-applied after the change is complete....
Not sure why air-con makes it worse though, although it is a fair load on the engine.
Does the problem seem as bad if you have headlights/screen heater/fans all on but NOT A/C? Might be worth ruling out the battery as there have been a number of bad batteries of late. Also try disconnecting the battery and reconnecting to force a reset.
At least you are still under warranty!
