That is a lot of miles. I am guessing you have kept the car well maintained. You are correct about the Idle Air Control Valve, (IACV).
With that many miles there are a few possibilities that could be causing the problem.
The throttle body, IACV and Exhaust Gas Return, (EGR) are probably really dirty and they probably need to be removed to clean them out properly. You might be able to save the IACV and EGR by cleaning them. The IACV is usually found on the intake manifold near the throttle body. Anti-Freeze runs through it and it will have an electrical connecter as well. The EGR can cause problems from the dirt and, "Tar" like stuff that builds up in the intake tract over time. It should only open at highway speeds because it will cause idle problems at low speed or a stand still. When you clean them be spray out from the back or opposite direction of flow as you do not want contamination to get deeper into them, (brakeclean is good or carb clean if it is really bad). The IACV will have a screen and O-Ring in most cases and the EGR a gasket. Replace those and look for vacuum leaks around the intake. With the mileage and age of your car, rubber and plastic becomes brittle and cracks open causing vacuum leaks that cause idle problems.
A tune-up and replacing the O2 sensor would be a good idea as well.
If this does not work, there could be a problem with the Torque Converter in the transmission. I am inclined to think it might be the major culprit because the problem happens after driving on the highway. The torque converter will stay locked after driving highway speeds and not loosen up and cause idle problems.
I hope this information helps.
Let me know if I can be of further assistance and how things turn out.
Wednesday, February 2nd, 2011 AT 10:18 AM