Flashing Check Engine

I have an 99 buick lesabre and the check engine light is flashing. When at a stop light or in idle the car kinda jumps or should I say feels like the engine is sputtering. Then when I apply gas it like jumps harder til at a constant speed but it still feels heavy, is a bit louder and the light is still flashing. The car has approx 86000 mi. Please give me some type of info on possible causes. I have been screwed over by 2 different shops previously on a different car and right now I have little or no trust for the mechanics in my area. At least with some ideas I can understand them more clearly and won't get taken for a ride. :(