2001 4.6 hesitation/stumble under load

2001 crown vic police interceptor 4.6 has an issue when giving it more than 1/4 - 1/2 throttle. It stumbles. It's also very slow to get up to speed even if you baby the throttle. It shifts and o/d works. Twice now it has seriously stalled in the middle of driving, giving it barely any throttle would cause it to start seriously dropping rpms, but held the idle fine. After shutting it off and waiting a bit it fired up and went back to the original problem both times. Was told the MAF, tps, fuel filter and injector gaskets were replaced. Air filter cleaned and the engine seafoamed. Previous owner also had diagnostics done and the only codes were lean and rich, possibly because it's straight piped no cats. Any help is appreciated

Also it will change rpms when idling, but it's very subtle and doesn't happen often.

Replace the IAC.