Isn't there a Jaguar group which can be more help to you?  I don't think
many Audi owners are that knowledgable on Jags.

 Hi, maybe you can help me.
 I own an XJS V12 (1988). Car is in very good conditions and has low milleage
 ( 80000 km, about 50k miles).
 I do live in the Canary Islands (Spain), so climate is mild (average temp
 21ºC).
 This winter is a little bit colder (13ºC in the moorning). When I start the
 car in the moorning and it's cold (maybe hot for you...) it starts perfectly
 but at idle speed it varies roughly the rpm's (the engine does not stop but
 rpms go about 500 higher and lower than 800). It also makes a lot of smoke
 (@ both pipes) with a white colour (not a clear white but nearly a light
 grey to white smoke). After a few kilometres the idle speed becomes stable
 and the smoke dissapears (during driving, before it dissapears, if I drive
 at constant rpm's I can feel a light tendence to decrease/increase rpm's).
 I guess I changed the air filter, sparks, oil and oil filter one year ago.
 Can someone help me before I send my car to the garage?

 thanks in advance

 I've seen this sort of thing before.  Luckily it's easy to fix.

 Drive the Jaguar to the nearest Audi dealer.  Trade it for a nice, low miles A-8.  You'll notice the idle will smooth right out