Burnt cookies anyone?

Tonight is our KidzTown Connexions' Christmas party and I've been home this morning baking for it. Now, I am running two sheets at a time and they are all on a timer, but every second batch is burning! What is the deal with that? They are the same sheets I use for everything, the same dough, same position in the oven, same temperature and same amount of time, but still they're burning! I don't burn things! Dylan ate one and said "what's wrong with this cookie?" (That's because he's never had a burnt cookie!). I don't have time to rebake cookies and tomorrow I'm supposed to be doing my perfectly white shortbread. I did realize this is the first Christmas I've baked with this oven, but I've baked during the year with it. Maybe it doesn't like to be used this much. Suggestions anyone?