I also had a similar issue on my 2010 C4S. When I first got car it would idle erratically +- 50 RPMs (everyone said it was normal, it’s not!) Then got worst over the next few months sometimes as bad as +-200 rpms. This seemed to only happen when car was cold (like first 20 minutes into drive.) No codes were thrown. Throttle adaptation from dealer didn’t help.
My dealership searched for 3 days trying to find the cause from MAF to Coils to Ox sensors and everything was fine. They even re-flashed the ECU to a newer version and nothing helped. I suggested the throttle body (my first thought, and their last) and they happened to have one in the next bay off an engine rebuild they were doing and magically they fired it up and ran perfectly! Dead solid stable idle. The throttle body developed some strange play in the servo or gearing or something.
One thing that I had that you may want to look closely for is that this “erratic-ness” would be felt at low idle input. So if I was steady on the throttle pedal (15%) the car would pulse like the RPMs would at idle.

I wish I went with my gut and just bought a used TB on eBay for $150 to try it before giving it to dealer but oh well…
Hope this helps!

 

Pull off the TB and hold it up to the sun to see if there is any light coming through. I had a similar problem that was a microscopic leak in the TB seal from cleaning it. Also try a bottle of sea foam . Forget the techron. If nothing helps, pull plugs and check all gaps.