problems with 88 sable

just got an 88 sable gs, 3.0L V6....starts fine but rpms kinda jump a little then settle down to bout 900rpms...when u go to give it gas it wants to die and sometimes the lights even dim, but it seems to pick itself up and takes off....then once i keep the throttle at a constant position the "check engine" light will come on, give it gas or let of throttle and light goes off and car acts normal...while the light is on car hesitates..... wonderin if neone has suggestions for me and if there is a way that i could check what is causing the light to go off or get some type of reading from the car?

thanks in advance.....

aim me at : JoshTheJoker if u have ne suggestions also