You may be getting an intermittent track accross the plug leads. Re-arrange your plug leads so that they take a different route and try to separate them. If the problem goes away, then you have your answer and you require new leads.
BTW, only just got your email. I have a set of newly refurbished Sebrings that I am not sure what to do with. (Although one now has slight rim damage due to some stupid Italian who did not have a fcukin clue on how to fit tyres. Cnut!)