A classic case of ignition arc-over due to moisture. Generally corrected by
replacing all high tension cables and the distributor cap, which may be
cracked and/or dirty. A general cleaning of all ignition components and
possibly a damp-proofing spray of some sort would be a good idea. It is
possible for moisture to get into other electrical connectors and components
and cause this, but the most likely cause is as described. Hope this helps.
Don Young

"" wrote:

Where I would begin with a problem such as yours is through covering
all electrical connections on the ignition system with a petroleum
jelly like vasoline.  Petroleum jelly repels water so any rain that
might be getting in would be blocked in effect.

You can be quite liberal with the application but a covering is all
that is required.  All connections especially areas like dizzie’s can
be coated.  I have found on other brand vehicles that often moisture
inside the dizzie is the cause.

Good luck!

Paul and Karen,
   The XJ40 is known to have poor electrical connectors in the engine
compartment.  There are owners who have "cleaned" the inside of the engine
area only to find themselves unable to start the car for days.  I my own
experience, I went hours with a hot engine before the connectors dried out
enough to restart the first time I degreased the engine bay.  Most of the
problem comes from the connectors on the front right part of the engine bay.
   Slathering Vaseline or some other type of grease will make for an
unsightly mess under the bonnet.  Over time the grease will melt and drip
and you will lose the effectiveness, the problem will return and you will be
hating the day you "Vaselined" up the car.
   I suggest that you go through all of the connectors, dry them one by one
and give them a shot of WD-40. WD-40 was originally designed to improve
electrical contacts.  I rarely ever use it for squeaks since it evaporates
rather rapidly.  Once you have done that,  after each connector you do,
start the car to make sure all is well.  If the car does not start -- you
have found the problem connector.  If it does, you have protected the car
from future moisture problems at that connector.  Then, as extra precaution,
I wrapped my connectors in plastic and covered the assembly with black
electrical tape.  It is an inexpensive, effective and clean solution to the
problem.
  Finally, the problem may be with the distributor cap as others have
mentioned.  With a 1994, the chances are you may still be running on the
original cap.  I suggest you go out and get a new one.  Caps are designed to
not let moisture in and if yours is letting in moisture on a damp day, than
there is a problem.  Change the cap, clean the base of the distributor and
if you still feel the cap may be the problem, seal the base with a clear
silicone caulk. If you use that old fashioned tool -- your finger -- you can
take a small amount of silicone and simply fill the space between the cap
and the distributor base with a clear smooth nearly invisible and
water-proof line of caulk.

Webserve