Tuning (PLEASE HELP!)

[HotRoddin]

Originally posted by King Big Ben
And are you sure its the headers because a friend of mine said its probably not.

I don't think he ment your headers were causing your problem ... he just ment you needed headers with the set up you have. It sounds like a timing issue to me. Could be ignition timing or it could be valve timing. Double check and make absolutely sure your firing order is right. A vacuum guage reading would be helpful. Also check that your accelerator pump is working, that will for sure make it cough and fall flat when you stomp the throttle.
You really need to join a local mustang club or car club ... there will be lots of people there willing to help you.