Check your plugs and wires. Look the wires over good for looseness on the coils or plugs or maybe even bare wire on the headers or whatever. Check your plugs and the plug gap. May need new ones. How clean is your air filter? It'll run rich if your air filter is dirty. How's your air pressure in your tires? That can also make a difference. Run some fuel system cleaner through it such as for the injectors, etc. Are you consistently getting your fuel from the same station? You might change stations. Maybe getting bad fuel, water in it, etc.
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1996 Mustang GTS