1998 Chevy Camaro Runs rough

Runs fine at idel or above 50 miles an hour, but between, when you accelerate car misses, runs rough, and lacks power.

Already had it tuned up, and mechanic looked it over but problem still exist.

I'm thinking maybe it is the idel aire control? I have some other ideas too, but what would you recomened the most likely problems it could be?

Thanks.