Recently I have installed some new components to my car, which are NGK Iridium spark plugs, and a K&N SRI. Both are great additions to the car, but on thing I have noticed recently is that when I am at a stand still the car will "stumble" or make a strange vibration. I've consulted a Chilton cobalt manual to find out what the issue may be. And so far it looks like it could be a few things, but nothing for sure. One that did cross my mind was the spark plug gapping as NGK's site said to make the gap at 0.52 in. for my '05 ls. I first I thought that seemed kind of like an extreme gap, test drove it, and it seemed fine. Today I gapped them at the 0.40 in. (stock gap setting), and the car will still make the sudden vibration repeatedly. One friend said that it may be the clutch slipping or it lost a tooth, something like that, but still not quite sure. Another that crossed my mind was the fuel filter, as my car has about 37k, and may be due for replacement. Any information or advice that anyone can give would be great. And I did try searching in the forums already for this stuff. Thanks and glad to back on the forums.