94 Sable LS high idle

I have a 94 Sable LS with a 3.0 with 123k. The car has developed a high idle problem that is intermittent. If I drive it on the interstate at 65 or above for say an hour it will be idling at 2200 RPM in neutral. If I shut it off and start it back up it's fine.
I had it in a shop on Friday and because I shut it off while waiting for them to take a look, we couldn't get it to act up. The could find no codes and had no idea where to look.
It did go into a high idle today on a short trip around town, about 1800 RPM.

try cleaning or replacing the iac valve.