My Car Stutters After Sitting In The Rain
It's common to get water in the plug lead area in the head (under the 'Elise' cover and down the plug tubes) and in the distributor housing. This evaporates when warm causing the misfire to go. There a many treatments, a fairly regular one is judicious use of silicone to seal areas that may leak. Some people have better results with Magnecor leads as they have a seal around the top of them that stops the water getting in. Another method is to fit the engine cover from the S2 Elise to the S1 with a bit of fettling.