Missing when over 30mph.

I have a friend that just bought a 99 GT few weeks ago. He says he has a problem with the car missing when it gets up to about 30mph. He said he has replace all the plugs on the drivers side and two coils on the drives side also. But the cars still misses pretty bad . I told him to go ahead and do the rest of the plugs and coils and see if that helps. I will see him thursday morning at work, but in the meantime have any of you guys have any thing else he could try or it you have been through before? Thanks in advance.