I have the same problem as Aaron had, my GT idles smooth then it dips and surges.  A few times it quit completely.  FOMOCO said it should be running on Premium.  I've tried this and it still continues.  Is the problem the Idle Air Control Motor?  I couldn't find one through LRS.

 Thanks, Atomic 

 What modifications are done to the vehicle? This is very important on helping determine what could be wrong. 

Some of the sensors and other things to check to check are :

1.  Air Charge Temp Sending Unit (click on the text to vist the link)

2. Mass Air Meter. It could be dirty or if you have installed a cold air intake make sure all connections are the piping are good.  

3. Throttle Position Sensor 

4. Other things include: Coil packs, spark plugs, injectors

These are just a few things to check from my own experience with mod Mustangs. I hope this helps you out. 

 JJ:

 The car's engine is bone stock.  The exhaust has an "H-pipe", no cats, O2 Sensor has been certified and the car passes emissions for air quality.  Flowmaster American Thunder dual exhaust, 3" pipes. Standard Fram paper filter, no aftermarket Cold Air intake.

Atomic 

 any check engine lights? how long has the car done this (was anything replaced or changed when this started)? I would start checking your sensors. If this doesn't seem to help you really need to have a professional machanic look at the vehicle. 

No engine lights...Ford put the car to the computer and didn't find anything wrong.  Fuel filter has been changed and I buy Chevron or Phillips premium gas.