XJ V8 sport cutting out whilst driving
 

Hi I have a issue with my 98 XJ V8 sport I can drive around town all day with no issue but once I get up to 60, 70, 80 mph cruising speed and ease off the throttle, sometimes the engine will stall comes up on the dash engine stalled and I have to put the car in neutral and restart whilst rolling or completely stop and restart. Once restarted I get a plume of smoke out the back end and then she is off and running again. It has stumped me I took the car to r.n Upton jag specialist in water orton they put it on a diagnostic machine and guess what no faults. They changed the crank sensor and a few pipes at a cost ! But still no joy as I got 3 mile down the road at 60 mph it cut out again ? They are clueless as to what is going on but still have the car at the moment.
Can anyone help please?
Regards
John

Wrong forum. Will have to scroll down a bit more.