cylinder 3 misfire

So for the past few months, when it rains, the next day when we start our 2004 jetta the check engine light first flashes, then is solid after a few minutes.  The car rattles and runs really hard.  Have brought it to several garages, they say it is coming up as a cylinder 3 misfire, after investigating, they both have found no spark from the ecm and changed the ecm.  Two days later, same thing happening, brought it in again, changed out the ecm to old one, changed wires to ecm, etc.  4 days later, same thing happening.  Anyone have any ideas?