I have followed this very informative thread with interest. Our ZT260 also has a very slight misfire on pickup when cold but on the Autobahn appears ok. It is in the Rover/MG workshops for the time being for other repairs such as the horrible rear window clips leak which allows literally pints of water into the boot wheel well area. I will get back to the forum to report on the cause of the misfire which could well be the coil or wetness around the plugs as already mentioned.