Nissan Altima 03 chokes and the service engine soon light is on? HELP!?

I have my Nissan Altima 03 2.5s for about 3 years, runs perfectly, but since a week ago the service engine soon light went on and off, mostly flashing, and the car chokes mostly when Im stopping behind the stop lights! Took it to a couple different places for diagnosis they said its from the fuel tank, they poured some kinda stuff in there, they said it will be fine when I drive on it for a couple miles, but still on and no change! the manual said the tank cap was not tight enough but checked it and it was fine... so dont know what to do! its kinda scary to drive it since its choking alot! any idea?