Idling high after long drives

I just drove my truck over 3,000 miles on the last 2 weeks without a major incident. However, I noticed it likes to idle high - around 1800-2k RPMs - after long drives on the highway, like when pulling off for food etc. Then, after sitting for 5-10 minutes and starting again, it suddenly idles smooth at 600 RPM.

The starts also seem quite variable - sometimes it will crank a bit longer, and/or sputter slightly before starting up, kind of like a mini-version of that it did when it had a bad ECT sensor, Other times, it will fire right up like a Cummins diesel. there doesn't seem to be any rhyme or reason to this in regards to the engine being being hot or cold, etc.

I don't see any pending or current codes, so I was just wondering if anyone had ideas on where to look first, or if this is just the nature of the beast with these old trucks.

 

Sounds like iac could be sticking. Regardless, take care with it when cleaning throttle body, clean it too. Plenty of info here and YouTube vids with little searching.