Rough running 912s2 during mag check

Fairly new 912s2 (8 months / 400 hrs). Maintained and flown regularly.

Intermittently during power check at 1640 prop RPM (POH rec), L only = no drop, R only drop = 100-200 drop and significant vibration.

Tried at various RPM. Appears that at 1000-1500 RPM, drop is normal on both mags (approx 50RPM), at 1800+ RPM drop is usually normal (80% of the time good), but at 1500-1800 we get this problem repeatedly (80% of the time bad).

Just came out of 400hr check including all new spark plugs - this happened intermittently before and after the latest check.

Maybe anecdotally, but it seems if I advance the throtte to a higher power setting, it is more likely than if I initially go higher (say 2000) then reduce the throttle. It doesn't happen every time and other flights have been normal, although I'm reluctant to fly now until resolved as something is clearly "wrong".

Suggestions please learned friends! 🙏