Hello,
Use the factory coils.
If you are replacing the coils with aftermarket coils, you will not get the revisions that Jaguar has made to correct known issues.
In your case, the coils are not sealing to the valve cover, and allowing water in.
The only alternative to that theory is that the cylinder head is cracked, allowing antifreeze into the spark plug well.
If you have rust built up on the spark plug threads (part inside the engine), you likely have a bad head gasket, cracked head or block.
I would like to thing they are becoming fouled by water leaking past the seals at the top of the coil.
If you post pictures online, drop the URL here so I can see what you have going on, otherwise, here are some local certified shops who will diagnose:
http://repairpal.com/jaguar-within-10-miles-of-gulf-breeze-florida-32563
Cylinders 1 - 3 - 5 are located along firewall side of engine bay .. You stated loosing some coolant with antifreeze smell .. Make sure there is not a coolant hose ABOVE that cylinder bank at some point that may leak coolant onto the cover .. Suggest removing the 'decorative' engine cover for awhile to aid in quick visual testing after each trip. (Ok to operate without cover in place!) .. When car is parked after trip, raise hood and LOOK around! .. Sometimes the driver/owner can discover the problem much more easily and quickly than mechanic due to having vehicle present for frequent testing .. Coolant leaks can be 'very elusive' at times . Good luck with it!
no rust color on threads. only on plug neck and coil cover around neck. engine runs great does not overheat. just misses when you first start it then smoothes out. almost like plug and coil standing in water then dries out.
Ok, it is more than likely bad seals on the coils. It is odd that it is only on one side... You might want to have this one looked at. Something you can do is leave the coils in, leave the engine off, and run water over the engine for a few minutes at a low pressure. Then, pull the coils and look for water. If no water, seek a professional diagnosis. There is not a good way for coolant to enter the spark plug well, unless it were coming from inside the cylinder, and the spark plugs are not sealing to the cylinder head. This is a common issue on VW and AudI models where the coil does not keep water and humidity out. Gulf Breeze, Florida... I'm sure you get a lot of rain and humidity. I do not recommend using any kind of silicon sealant. Also, make sure you are using enough dielectric grease on the spark plug terminal and in the end of the coils.