2000 Jaguar XJ8 Engine Idling Very Rough and Engine Shaking

Engine Performance problem
2000 Jaguar XJ8 V8 Automatic 95000 miles
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Hi,

I have a Jaguar XJ8 3.2 on a 2000 plate and it is idling very rough. When i am driving there doesn't seem to be a problem but as soon as i i'm standing still, the car starts idling very rough and the engine and the car begins to shake.. I have checked all the spark plugs and they are a firing correctly and burning evenly. I have also take it for a full diagnostic test and they can't find out whats wrong with it.

Is there anyone who has had this problem and has been able to fix this problem??

Thanks