Great. I am adding this to the list.
Stephen
While you are cranking the starter, do you see the tach needle move any? It should read 200-300 rpms. If not, the crankshaft position sensor is probably going or has gone bad. On the 6 cyl engine, the sensor mounts right above the crankshaft pulley and has a wire that runs up to the top of the engine where it plugs into a connector. The one on the V12 engine should be in a similar location. The part is not very expensive, probably in the $20-$30 dollar range but I donāt remember exactly. On the I6 it is easy to change, perhaps a 10 minute job. Hope this helps.
This post was from 3 years ago, its probably solved by now 
I saw that it was an old post, but thought the OP might not have solved his cars running issue yet!
Or if he did it would be nice to have the what fixed it info in the archive
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Iām getting back to the forum. The car is still sitting on my storage rack. I am at a point where I do believe I have the gremlin located. I just havenāt had time to get to it and donāt feel like paying someone else to do the work. I just picked up a 2001 XKR convertible and have a few other Jaguar I can drive for pleasure so it isnāt a priority. I will post up a full report once I have the car running and in tip top shape.
Be well my friends
I didnāt read the whole shebang here but if you have a replacement rotor, try that.
The rotor on this engine is somewhat unique, in that it relies on an epoxy potted resistor to SOLELY conduct the spark impulses from the coil connection (centre) to the outer end that serves the sparkplugs.
That resistor can, and does break down. Sometimes the degradation of the component can happen slowly, causing erratic running and symptoms ranging from lumpy idle/poor performance to total failure.
Total failure will prevent the engine from starting/running at all.
Very often the rotor is overlooked as a possible cause of engine issues because historically rotors are one-piece connectors from coil to plug and few mechanics know that the AJ6 rotor uses a resistor to bridge the connection,
That resistor is easily tested with an ordinary meter. Not sure what
value it should be but more than 10,000 is definitely a failure.
Jim XJSC 1984
Iāve got a failed unit on my desk which Iāll measure it later on, but at replacement cost of ~$10 at RockAuto, maybe good policy and easier to keep a spare in the trunk and swap the bugger out!
Somebody somewhere clearly considers it vital that there is some resistance in the spark circuit ā either in the spark plugs themselves, or in the HT leads, or even in the distributor rotor. I must say, Iād have to think long and hard before Iād design a rotor with a resistor in it. It applies to all the sparks, so it might even get warm.
My car has been tuned, gone through, and gone through again. The rotor was replaced but now with this info maybe the new rotor is bas. I will source a rotor and change it.
I will eventually conquer this issue. I just hope I donāt stroke out the minute I do. Like the rev. in caddie shack.
Originally resistors were added to ignition circuit to stop interference
with radio & TV, it is also a good idea to limit peak currents of the
spark to protect the coil etc.
Jim XJSC 1984 Brighton UK
Was it a common mod in the era of early electronic ignition systems (post points/pre coil on plugs) to use rotors with resistors instead of external condensers to achieve radio suppression?
Just wondering if this was an industry wide practice or just a Jaguar thing - most of my old mates had never seen a rotor with a potted bridge component and of course my younger gear head friends had no idea what a rotor was or did! 
edit: after a quick web search it does seem to be an across-the-board practice, not just a Jaguar thing.
I did find this, echoing @Jimandhelenās post:
*
> āThe rotor resistor was an early attempt at radio noise suppression. Same role as the resistance in spark plug wires and the spark plugs. No other reason. The resistor actually causes the secondary side voltage to rise slightly higher by a few percent before spark breakdown so the voltage overshoot is higher. Without the resistor there is less voltage stress on the coil, distributor cap, rotor, and spark plug wires. In addition, once spark breakdown does occur that added resistance will reduce the spark energy due to the current limiting it provides. For CDI use, the resistor should be removed and replaced with a piece of wire, or eventually the higher power of a CDI will cause the resistor to go open-circuitā - Contributed - Frederick C. Winterburn
AFAIK, all of that is incorrect.
Well possibly some, but certainly not ALL, e.g.
That ālack of spark energyā is why I had to have the car flat-bedded 25 miles last week - the failed component in the rotor immobilized the car.