Check your idle rpm in neutral with a tachometer. Should be 750 rpm if I remember correctly. Also, check timing. Retarded timing will cause this on a Ford V8. Disconnect and plug distributor vacuum advnce line, then with idle speed set properly, check timing. Can't recall the spec but you can find that in any tune up guide. c17hydro is correct that all vacuum lines and fittings should be checked for cracks and leaks, use some dish detergent around fittings. If idle changes, you've found a leak.
Sounds as though you have a vacuum problem...check your hoses to and from the carb...
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