Are you talking about while you are lightly cruising with steady pressure on the gas pedal? If its that, it could be your EGO settings.
Honestly i think its in your map. 18" is down at the bottom of the VE table. the very bottom bins are probably rich because you hardly ever use them. Look at your logs to see what bins you were drawing drom and check the ve number that was being added to confirm. Then adjust as necassary.
