Car shudders on start-up, possible spark plug problem?

I have a '97 Saturn SL2. When I start my car up after not driving it for awhile (say, overnight), it shudders a bit but then drives fine. However, when I run an errand that takes longer than a few minutes but doesn't take so long that the car can really cool down, the shudder is more pronounced on start-up. If I have to run a few errands, it does get to the point where I wonder whether I'll get the car to start at all. My car has been doing this to some degree for awhile, but I really noticed it the last couple of days, which were very hot. I'm not sure what's causing it. I had all the spark plugs replaced last year. Could they foul up that quickly? Could the heat have been a problem? Does how long I run the battery before actually turning the car on matter?