Steve is soooooooo correct. Many parts changers have no idea what the computer system is about.
First, look at the info you gave us.
No codes. This means, that the computer has not sensed any errors. When the ECM(electronic Control Module) is running, it examines each sensor Including the Idle Speed Controller for feedback. This is how the ECM can control this otherwise it wouldn't know what to do. So, if the controller is bad, it could / would normally sense a problem thus setting a code.
Since no codes are evident, Do as Steve suggests, start with basic tune up. Then move on to vacuum possible problems and such. Many times, when an engine is giving you simple problems like this, it could be nothing more than slight intake leak from a unconnect hose, or crack in the hose.
Another possiblitly that seems to look like is a low voltage problem. Sometimes, you can have a weak charging problem. With low voltage, while using the headlights and such for long periods of times, you can put a drain on the battery because the battery isn't getting enough juice to replace what is being used therefore, the condition can effect you during certain times. Let me clarify... During the day, you may not run your headlights, thus the low charging condition can adaquatly charge up your battery during this time as it isn't pulling much off the battery therefore your producing more than the battery is using, but during the times you have the headlights on, you are putting a heavier load on this system. When you first put on the lights, your battery can have a good charge but over a period of time of driving, it can start to weaken because you start to drain off all the charge it has stored then you end up with a lower voltage. Not low enough to set off any indicators yet, but this can cause the ecm to act erraticly and not have enough voltage to actually run some of the controller, thus giving you the same indication of a controller problem. What you might do is make sure your battery has a good charge, then turn on your lights and put a load on the battery with a load tester which many parts places use to check battery conditions. The battery and charging system should have no problem recovering from this and can show if you might have a charging problem.
The thing about computers on the newer engines are they can only point you in the direction in most cases as to possible problem areas. It's up to the mechanic to diagnose the cause but unfortunatly too many diagnose the sympton and miss the cause of the problem.
BTW, this isn't to say that the problem isn't the ISC, but before just replaceing parts, verify the other possiblities. One last thing.. most controllers used in controlling like a coil or a sensor, can be ohm'd out with a volt ohm meter. Many times, if you have no idea of what it should read, you can take the suspected part in to the parts house with you, get a new one out, and compare the ohms against each other and this also can give you some indication as to if it is possibly bad as well. You cannot do this with the ECM. That is a control module with many inputs and outputs and can be only tested in the vehicle with a schematic showing the in's and outs.
