Sounds like your typical surging idle problems. Many have had this problem with mods like yours, including myself. But, you can usually correct them with playing with the idle RPM. I have to run mine at around 950 rpm at idle to prevent surging. Remember to adjust your TPS after making changes to the idle screw, and disconnect your battery to clear your processor. Check that your throttle body is clean and I assume your mass air is calibrated for the 24 pounders.
Hope that helps some...
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fordgasm
CLICK HERE to see my 87 GT