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Car Revs High then Drops

i have a 2000 ford e-450, has been giving me lots of problems, i got the tranny rebuilt, changed injectors, MAF, air filter, fuel pump, fuel filter and now when it is in neutral i and push the pedal to the floor it revs high then drops then revs high and drops and keeps repeating this and you can hear some misfiring, there are no engine codes, i think its spark plugs because when i refill my tranny fluid it has a small leak in the filler and some leaked on the plugs, would this be the same symptoms? also when i was driving it the other day on the highway it just starting to lose speed even though i had the pedal to the floor and i had to pull over and drive it slowly home basically went 40km/h home and struggled up hills, thanks for any info

The surging at idle is very often due to vacuum leaks. With all of that work having been done I’d not be surprised if you have a disconnected vacuum hose or the like. That said, I have no idea what the part about you throttling at neutral. Are you saying that it will idle perfectly smoothly if you never goose the throttle like that?

Loss of power is very often related to poor fuel pressure - whether from a clogged filter or weak pump - or an exhaust restriction. I would check the fuel pressure at idle & under load and put a vacuum gauge on it to look for signs of exhaust restrictions.

But by all means if the plugs & wires are anywhere near due, then just do that first.

it idles perfectly, but when you push the pedal to the floor while its in park or neutral it will high rev then drop and start misfiring then rev high and then drop low and misfire, and when i drove it on the highway it just starting shaking every time i pressed on the gas and then eventually starting de-accelerating and wouldn’t accelerate after that, any ideas?

Yes, in addition to the ideas above, add cleaning your throttle body and idle air control (IAC) valve, checking your throttle linkage and your throttle position sensor.