That would mean that you either need to set your idle screw or your idle air bypass valve isn't working properly.
Set your idle screw by backing it out until you can slip a .010" feeler gauge between the end of the screw and the stop plate. Tighten screw until you can feel a little tension when you move the gauge. Remove the gauge and turn the screw in another 1 1/2 turns.
It should be able to idle at this point. If it doesn't, it sounds like the IAB is probably bad or not hooked up correctly.
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351W 89 Mustang GT Convertible