Chupes

I was drivin' around one day and noticed that while I sit in my running 1999 Cavalier, at a stop light for example, it sort of shakes or vibrates randomly. It's sort of like feeling a bump... bubump... bump bump BUMP bubump... lol. So I took it to the mechanic, and he said that my coils probably went bad. So he replaced them and the car was fine for around 3 weeks. Then, the same thing started up again, and a couple days later the "check engine" light came on. Before I take it back to the mechanic I'd like to have some sort of idea what the problem might be, just so he doesn't give me a line of bull. Thanks for any suggestions on what could be the problem!