I have the same problem. Van sputters when its idle (eg traffic light) and sometimes when it accelerates.Thanks for posting this information. I will take a look into my mpv's own sparkplugs although I have never washed its engine compartment with a water hose.As far as reseting the engine light goes, I'm pretty clueless. Sorry.
I figured out through research on this forum that all you have to do is disconnect the battery for a few minutes and that will clear any codes that has been set off. I did that and every thing is now back to normal.
I wish I had some input for your situation. How many miles are on your MPV? If you haven't ever washed the engine it is probably doubtful that you will have the same results that I did, but you never know? It may be possible for condensation to accumulate around the plug boot/s if one of them is dry cracked or something and if you have had alot of rainy weather?