2003 Mazda 6 Shakes when in gear at stop light

Shakes or Wobbles problem
2003 Mazda 6 6 cyl Front Wheel Drive Automatic

Getting a violent shake at the stop light when the car is suppose to be idling, it is "lunging" or jumping like the accelerator is being pressed. I have cleaned the MAF sensor, replaced the air filter and repaired a cracked air intake hose. The codes at Auto zone say really vague things, like fuel system pressure low (usually gas cap-was on tight), lean air/fuel mixture (ran injection cleaner through-runs better but still jumping). What could it be? Is there a throttle positioning sensor? Where should it be set?