Ford F-150

I have a 1987 F-150 4x4 with a 5.0, about 200,000miles. Sometimes when I push the clutch in the engine will rev but I can rack it up with the accelerator and the rpms drop or shut the truck off and start it back up and it will sometimes quit doing it but other times it keeps revving when the clutch is pushed in. I have replaced the idle air cotrol motor, new accelerator cable. Both me and my father have come to a stand still on the problem, any ideas will help.
April 24, 2007.

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