Hi Piotr,

From what I have read, it would appear that your pre-cat Lambda sensor is working fine and as already been said, it is reacting to what it is seeing. I cant see it being an ECU fault but I think the next step as has already been suggested is for a smoke test. Is the switching cycle the same at normal and fast idle? During the cycle, can you hear the engine note change? As a long shot, I would disconnect the EVAP and EGR to see if there is any difference.

An interesting problem and I wish you the best of luck.

Regards

Pete M