Strange engine problem

Hi Have an 1990 XJS V12 convertible owned for 20 years and am having a problem regarding the engine. Starts easily and settles at 10500 rpm then drops to 7500 at idle when hot, no sign of the AAV fluctuation (it is new). Car will usually run as normal but will suddenly drop revs down to zero, sometimes stalling but starts straight away. Hot starting no problem very occasionally will start from cold and start the dropping revs …It has never let me down . I have tried looking at the engine when dark no sign of arcing .all injectors ticking away. Any suggestions as to what’s wrong
My own observations point to fuel delivery or an ignition problem but am no expert.
Thanks Dave

Those neat round holes in the bonnet were made by the pistons departing the engine.


Check the fuel sump tank for crude and dirt blocking the filter.
Lots o info in the archives.