[jeffcoslacker]

Mass air flow (MAF), the one in the duct from the air cleaner to the intake has been known to do this. When it malfuctions, the ECM cannot determine volume of air entering the motor, and so cannot meter fuel properly. If you catch it in the act, tap lightly on the sensor body, with a plastic screwdriver handle or something like that, and see if it clears up. Conversely, you can tap on it when its running fine and see if the motor begins to break up as you do it. If so, you've found your problem.