I just wanted to post an update. It has now been warm for a month or so, actually have even had some pretty hot days, and my car has not exhibited the sputtering symptoms. I dont know if its related but about three months ago, in the freezing cold, the alternator died. I had it towed to Pep Boys, which I am now boycotting because of a terrible experience with them, but thats another post. They installed a brand new alternator and battery. Besides a couple of oil changes and new brake pads in the front I have not had any other work done to the car. I feel safe to say the mysterious sputtering has gone away. Maybe it was the alternator all along? I suppose you guys would know better than I would. Any thoughts?