For some reason when I am going over 5mph and I have the car in neutral it won't idle right. It bounces around between 800-1300rpm but once I am almost to a complete stop it will idle fine. This is driving me crazy trying to figure out the problem the idle air control valve seems fine and there are no codes.

 

ah the joys of 5.0 fuel injection. Mine did that, try cleaining the "salt and pepper" terminals behind the upper intake. Sometimes when those get dirty they can make your car surge. Your EGR can be bad, your BAP sensor could be bad, Your tps voltage could be off........ blah blah blah along with like 50 other things.

 

Its a 4.6 2v. The car is a 98 GT. I checked the tps voltage already.

 

do you have a cold air set up on it ? have you cleaned your mass air sensor latley ?

 

If you have mods post them, or is it all stock?

 

I have not cleaned the mass air recently.
The car has a S-trim supercharger(6psi), 24lb injectors, 87mm mass air, 70mm throttle body, chip, coil packs, headers, off-road H-pipe, mil eliminators, other than that the motor is pretty much stock

 

does the car idle just sitting still ? if so id have to say where ever you have your filter located needs to be covered more !

 

The car idle fine when it is sitting still. The filter is located in the bumper and was recently cleaned.

I am thinking maybe I should get the car tuned?

 

Absolutely get the car tuned. You may have a hole after the air meter allowing unmetered air into the engine.

John