Hi Dave @davejones
I had exactly the symptons you have described with my own 3.6 Cabriolet last year – Slow running seemed to be a random thing that the car either decided it wanted to do or not on a whim. Made driving it very hard and unpredicatble – sometimes needing to be sat at traffic lights bliping the accelerator like a boy racer to keep it going and others the car being perfectly behaved.
We swapped a the same sensors as yourself, with similar results to your own – it would work for a while then start messing aroudn again. Then one day the car wouldn’t start at all -dead 🙁
Subsequently we HAD to change the ECU to get the car going and borrowed one – and all the random yet persistent slow running issues went away. Replacing the ECU was not on the list of things to try, but I wish it had had been right at the start.
I’ve met a couple of other folk who have had similar problems and it does seem to be that if you have this random slow running issue that its cause by the ECU – when it’s a consistent and persistent slow running problem then it seems changing the sensors does fix the problem.
I’d be interested to know if others in the Club have had the same experience.
Hope this helps,