Orson:
I'd be surprised if the main dealer service did anything but touch the bare minimum on a service. Most dealers are frightened to death of the V12 and push them back out the door as quickly as they can. The symptoms of bad vacuum advance are not particularly obvious, apart from a poor fuel consumption, as mentioned in the link in my previous message. A good modification is to disconnect the 15 minute timer that disables vacuum advance below a water temperature of 45 degrees C. There seems to be no real reason for this system, and removing it has no effect on the car, apart from improving fuel consumption in the first 15 minutes of driving. The auxiliary air valve does a fine job of regulating the idle to the engine temperature as it is warming up, and the ECU sensors adjust to air and coolant temperature. I suggest you download the free book "Experience in a Book" by Kirby Palm. This is a distillation of several years of information from the Jag-Lovers XJ-S forum and is probably the most useful book on the XJ-S you can get. It's available in PDF and MS Word formats. There is an extensive section on the distributor that should be required reading for all Jaguar V12 owners. It's at: www.jag-lovers.org/xj-s/book/
Happy reading!

I've just returned from 5 months in Texas where I did quite a lot of driving and I found the following:

Speeding isn't really socially unacceptable. A similar mix exists to the UK, some drivers obey the limit to the letter (or number rather), most do an indicated 10-20% more thereby breaking the limit a little but not really risking getting caught, and a few drive very fast weaving from lane to lane as is the way on US highways.

I was horrified by the extent to which drink-driving IS acceptable, socially. Lots of people are prepared to drive to a bar, have half a dozen drinks and drive home again.

The standard of driving seemed to be a lot lower than in the UK. The driving test appears to be far easier and almost anyone is granted a licence after some "drivers' ed" at the age of 16. I got frustrated by rolling-roadblocks that seemed to occur on the highways due to 3 people all travelling slowly side by side - there is no lane-etiquette on the highways. Mobile phone use was rife and signal-usage suffered as a result. Drivers would frequently slam on the anchors and turn left (across the flow of traffic) without signalling at all, simutaneously talking on the phone. Hanging in another driver's blind-spot also seemed to be a popular sport. Drivers seem to often be unaware of what's going on around them, pulling off manouvres regardless of whether or not there is someone in the way. If anyone's played GTA3 and tried to "obey traffic laws" for a while, you'll understand what I mean.

Basically, all the irritating things that UK drivers do, but moreso.

The trick is,

if you have the rubber stops under the bonnet on each side. Get someone to help and open the bonnet, then unscrew each rubber stop by a turn and try it. If better then turn an extra half turn.

Thanks for your help Peter. I've found out what it is. It's the Air flow meter (or wiring) ! I unplugged the meter connector on the air filter intake (disabling it) & took it out for a test drive. It was absolutely fine (although idling was a bit lumpy - but that's to be expected) ! I'm so chuffed that it's been resolved ! Thanks very much for your input anyway, it's was apppreciated.
Now all I have to do is diagnose if it's the wiring to the meter or the meter itself ... I don't really fancy forking out £160 to gsf, but if I have to, then so be it - it's worth it just to get the performance back.

Cheers everyone ...