[rv8r]

My engine always oscillates at idle RPM (and up to 1300 RPM), it's and steady up and down, every 2 seconds it goes down as if one magneto is shut down, and then comes up again. I tried to run it one one mag only, left and right, but it's the same on both, only on left or right mag. My Dynon EMS shows nothing abnormal, fuel pressure OK. No difference with electric fuel pump on, carb heat on, leaning or anything. In flight it's all OK, but I wonder what could be the reason for this strange behaviour?? Any ideas out there? Maybe the Ellison TBI gets some strange airflow (I re-shaped the airbox a little to clear the cowl but did it symmetrically on both sides to get an even airflow, but who knows...??)OH, I run on AVGAS, and it's the same with a cold or hot engine.
Thanks a bunch, Dirk