Thank you all for the replies!
The exhaust is on my list since some time now but I didn’t have the time to unmount it.
Looks like i’ll be busy this week end with the jag!
I’ll keep you updated once done 
Sorry for the late update , my is suite time consulting lately.
After working yesterday on the car, we’ve ‘‘solved’’ the issue : one of the cat converter is completely clogged.
No more issue while running without the exhaust line (only the the manifold).
Thanks you all for the time and advices ! 
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