I had a similar sounding problem to this. The car would hiccup and then be fine then a day or so later would throw an emissions code. This happened a couple of times, I changed the plugs from bcr8es to iridiums and I've not had an issue since. The only thing I can put it down to is the original plugs fouling as they were to cold for my general driving (like a granny). The iridiums are much better from cold first thing in the morning as well
