Why is my engine sputtering?

Hello - I was on my way to work and, while sitting in traffic, my engine began to sputter. I turned around and went back home and it seemed to sputter some on acceleration, then when at a normal speed seemed ok, but sounded rough. When I turned the car off and opened the door, I smelled something that I've never smelled before - a borderline burning smell. Oil level is ok. It sputters more in Drive rather than in Park, but sputters in both gears. Most parts on the car are new, including the timing belt and water pump. Any ideas?