From what you describe, it sounds like your auxiliary air valve may not be working as it should. This valve provides extra air and fuel from the cold start valve during warm-up. This raises cold idle speed which then gradually drops as the valve closes off in response to the rising temperature of the engine. You may be able to remove the valve and clean it with carb cleaner if it is sticky. I recall a Jag site that had some repair tips for this valve. It was also used on the V12 jag engine. Also, be sure to check the operation of the cold start valve and the thermo-time switch which work together to dump in that bit of extra fuel.

Search this site for other tips on maintaining the aux. air system, and get a service manual if you do not already have one.

Good luck,

230/8