Weak RPM while idiling

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.

The labor should be around 2.0-2.5 hours. So, based on a Labor Rate of $80.00/Hr, it will be $160.00-$240.00 for Labor.
The valve and gasket should be around $250.00, although that depends on where you live, i.E. Costs are higher in Urban areas as opposed to Rural.
You also have to take in account the shop costs for disposal of hazardous waste, fluids, cleaning agents, etc. This will be around $30.00.
So, that is a high end estimate of $520.

Let me know how it goes and I will keep an eye on this post if you need any further assistance.

That does sound like it is failing. The bearings and/or bushings inside are making the noise because the different types of materials expand/contract at different rates. Sort of like if you put a bottle of water in the freezer, it breaks the bottle because the water expands more then the container. Ice is kind of an exception to the rule, but I hope it relays the overall reason for the noise.
Try some Power Steering treatment made by Lucas Brothers. It works well to quiet pumps and can even fix leaking seals. It even guarantees that after using 2 bottles.
Be sure to drain out some Power Steering fluid to make room for it. Overfilling will blow seals. I take the lower reservoir hose off pinch it with my fingers and put it in a 12 ounce water bottle. The Lucas Brothers stuff comes in 12 ounce bottles so you can measure the P/S fluid and not empty too much out.

Thanks very much for the compliment. It is always great to hear that I helped someone out. My boss is a great guy and I told him about the same thing you said about how he created a good thing with this site. So, I was a little astonished when I read your reply. Even though we struggle with communicating through text, it still works. The Internet ran out of IP addresses, (or Domain Names or whatever), and I heard that the analogy of the amount of new numbers was, "If the old amount of numbers were the size of a golf ball, the new numbers are the size of the sun".
Kind of blows your mind that with all of that, none of us are really strangers.

Take care and let me know if you need anything else.

Dr. C