Sounds like a vacuum leak; lean running. The smoke is never a helpful
clue as it could truely be steam. the jerking is an indicator of
running lean.

On the v12 the intake manifold works quite well unless somebody at
somepoint removed it. If the intake manifold is not torqued in steps,
using the right tightening order, these intakes will tend to leak.

Distributors have been a major problem on v12. Prone to allow water
inside and damage the advance mechanism. Rotate the rotor with your
hand and let go. It should quickly snap back and should have no
slack/wobble when gently wiggled.

1. Change your fuel filters.
2. Check for loose, missing, or cracked hoses.
3. Spray carb. cleaner around the mating surfaces of the intake system.
4. Have fuel pressure regulator checked for proper idle and driving
pressure.
5. Have exhaust checked for back pressure 0-3 psi is ok, anything over
that might mean a failing catalyst.
6. Clean temp sender connectors.
7. Check fuel injection amplifier.
8. Look iside the distributor for water damage and possible advance
mechanism issues.

Here is a great document to download from Jag Lovers. Every XJS owner
needs one. It is a Word document I believe. I can convert it to a PDF
file if you need so.

http://www.jag-lovers.org/xj-s/book/XJS_help.zip

Ciao,
Blake