A bad O2 sensor will cause the car to run rich. Old stang use to stutter and would lack a ton of power below 3000 RPM. Changed O2 sensor, problem solved.
Dump the codes and see what the computer says is wrong…
Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.