Sorry Peter, sounds like a dealer visit is in order.....from what you describe I would suspect water in or corrosion of the Lambda wiring plug where it connects to the main harness. As the system is looking for a reading in the range of 0.9v-1.1v it does not take a lot to corrupt the signal, you did not mention the car going into 'Limp Mode' so that's where I would start....fingers crossed for you.
