Ok i own a 1999 v6 mustang and everytime i go to stop my car starts to idle at like 400 rpm and it will go back up to like 900 rpm and it will keep doing it like its not getting gas or something i have replaced my fuel filter and sometimes it will try to die and if i try to give it gas it suputters really bad its ridiculous can somone elp i have also replaced my spark plugs and sprk plug wires.

 

Is the car throwing an idiot light? Get the OBDII codes read to see if the PCM is complaining.

Could be a vacuum leak, which can cause a hunting idle. Pop the hood with the engine idling and listen carefully for a hissing/sucking noise - it may be difficult to hear. This indicates that you have a vacuum leak that could be a split hose, broken connector or simply a hose has fallen off. A trick that mechanics use to find vacuum leaks is to spray ether (starting fluid) or propane around the gaskets and vacuum connections and if the idle increases, you've found your leak.

Any chance you got a bad tank of gas? You might try a bottle of fuel injector cleaner as well.

 

If you haven't found your prob. yet, it is a good bet you have a bad IAC valve. It is attached to the side of the throttle body, and controls air when the gas pedal is not being used. It is prob. dirty, but it can't be cleaned, because it is a sealed unit. They are not real expensive, and relatively easy to replace with intermediate mechanical skills.