It's a long shot, but I had an elusive check engine light that the dealer couldn't track down, and it ended up being an aftermarket PCV valve I bought at Pep Boys. The thing was 2 pieces (even though it looked to be one piece), and was letting air leak into the intake. A $3 fix that ended up costing me $75 in diagnostic time + time/effort to keep taking it back to the dealer + LOTS of frustration. It's worth a check, especially if this started happenning not long after a PCV valve change. Remove it, grasp the base of it, and see if there is play in it when you pull/push on the elbow.