1997 Ford Mustang My Engine stutters right around 4000 RPM

Ok so tell me what all have you replaced? What have you checked? Do you have a digital multi meter is we need it? Is there any performance mods? Get back to me with as much info as you can and we will go from there.

Ok I would like to test the TPS. To do that you will need a paper clip and a digital multi meter. Now at the tps you will need to use the paper clip to back probe the gray wire with the white stripe. Now hook up you meter set it on volts. Now turn the key on engine off and check voltage should be around.8 volts. Now slowly open the throttle the voltage should increase as you open it, it should increase with no drops or glitches and should read about 4.5 volts at wide open throttle. Get back to me with what you find. We will see what to check next. I would like to know if this does this all the time or does it seem to do this more when the engine is cooler or warmer? Did this start after any of the power adders? If not then when did it start? What spark plugs did you install? I have done a few bolt ons with my 98 and have more to do as money permits. Anyway get back to me and we will see what to do next.

Ok I would also like you to check the air charge temp sensor and the coolant temp sensor. Just simply unplug them with key off engine off and ohm them. Do this when it is cold and give me the ohm's and the temp at the time you check it. Now if you have a way to read the engine temp then do the coolant temp sensor when it is warmed up too and give me ohms and engine temp with that test.
Here is a pic of the temp sensor it should be near or on the thermostat. The air charge temp sensor looks very simular except it is either in the intake tube or on the intake manifold.

Ok as I have asked check the coolant temp sensor and the air charge temp sensor. Get me the ohms of those sensors and the temp at the time you tested them. Do this when the engine is cold and give the air temp at the time you checked them.