I had a hesitation and loss of power recently and it was a very small leak in the valve cover gasket that had allowed a small amount of oil to pool up in the holes where the spark plugs were closest to the windshield (because the engine is tilted at a slight angle leaning backwards, which is normal) If this is the case, you should be able to notice if when you take off the valve cover gasket on the driver's side and remove the spark plugs, you will notice some oil pooled up. Not much but enough to do what you are describing and throw off performance of spark plugs.
Like Roberto mentioned, at about $3 a piece for the OEM Bosch copper-plated spark plugs, it's the lowest-hanging fruit as far as time and expense, if it does happen to fix the problem. If it is a small amount of oil, I wouldn't worry too much about replacing the valve cover gasket because the oil you are seeing pooled up in there (if there is any) is what has accumulated over tens of thousands of driving miles. Unless it's a lot or happens again right away and then you probably need to replace the VCG.
And also I mention this because none of the electronic or DEM diagnostics done by the dealership picked up the actual source of the problem.
Last edited by manuelwindows; 11-09-2012 at 12:36 PM.