2 things.

Our cars, especially modded do tend to misfire at idle. Though enough to cause it to idle rough sounds like something more.

2nd, if the car is misfiring, your check engine light will flash on and off.

What plugs did you put in, and what did you gap them to? Are there any mods done to the car?

How old are the plug wires? Did you make sure that they were all contacted with the plugs well?

Does the car run okay driving, or does it misfire while driving too?