The codes usually won't erase that easily on an OBD2 car, I'd still get the codes checked just in case, as previously mentioned at AZ or something. The MAFS or O2 sensor don't really make sense for this problem, sounds more like something with the throttle motor. If it happened right after the plug change, I'd concentrate more on the air intake leak problem. I'd look at that duct tube (cross-over tube that runs over the valve cover)over on the drivers side of the engine where it goes into the throttle motor, see if the tube dropped into that rubber ring properly, not pinched or part of the rubber folded in.
You only notice the problem at idle?
Can you describe the problem a little more clearly? How much does the idle fluctuate, and is it doing it very frequently, or like every 5-10 seconds or what?

Gilly