What's wrong with my car?

This has been happening off and on for a week and a half now. My check engine light came on and every time I stopped and the RPM dropped to about a 1, my car shakes then rattles and then it sounds like the engine pops, but it feels like the engines going to pop out of the hood. I thought I might have gotten bad gas, so I put fuel cleaner in it and the shaking and such didn't stop. I had a mechanic look at it and he said the check engine light is a dummy light and I guess it didn't rattle or shake when he was driving it. Any suggestions?