1999 Denali sometimes revs in park. Between 400 rpm and 1000rpm

[Panner99]

Hello. So I'm having this intermittent issue. Happens maybe once a week. SUV is warm. And if I start it again it will rev up and down. SUV is in park. It revs between 400 rpm and 800 rpm. I cleaned the IAC. I put a new PCV valve in. Checked for a vacuum leak (all is good). Not sure what to do. No check engine light. It drives fine.

It will rev like this until I turn the truck off and then if I restart it it will not do it again. It will idle properly.