I recently purchased an RV-6A and I think it has this same problem. I took a video of the stuttering at idle after it has been run for about an hour:
https://www.youtube.com/watch?v=pzI3RUR9KNg
Is this what other people in this thread have been seeing?
Note the tach is fluctuating about ~50 RPM but there is no fluctuation in the fuel pressure. Pump and mixture have no effect on this. My stuttering occurs with less frequency/severity in idle flight but on the ground it is pretty bad.
My -6A has no gascolator, an in-cabin electric fuel pump, and has the blast coolers for the mechanical fuel pump. Given this, I think the problem is probably after the fuel servo. All of those lines are wrapped in heat shielding up to the spider.