Since no 1 lead seems to cause problems, could it be that the cap has a crack or a coal/dirt 'bridge'. It could cause spark to jump sideways and find it's way to the other plugs and 'upset' the charging of the coil. Just a thought... Changing the lead/cap and cleaning the head between the plugs could help in such a case.
-99 4/4, 1.8 Zetec, Indigo Blue, 2-seat, Suplex RS
-04 Citroen Pluriel, 1.6 Sensodrive, Met Orange
| 
