it's most likely egr. the egr vavles on these cars are an electric motor, idle air control valve type, and yes they are a p-o-s. Same ones used on some hondas as a matter of fact. the pintle on them gets stuck usually half way closed, causing a slight over flow of oxide gases and your ses light to come on. since egr only works at stable crusing speeds imo highway ect. the light my go on and off. if you open the car up on the highway for example, and bring the car back down to like about 60 or lower (say exiting the expressway) you may seat the pintle properly and the light will go off, untill you reach and maintain a steady crusing speed once again, where the valve has to open and close rapidly, and u get the light back up again. cause the valve dosent seat properly or, stays open/closed. the egr is very simple to test, and change, but it is expensive, $155 i believe ac delco.