I did the conversion on my Arrow and had the same problem. I had it
dynamically balanced and rotated 180 and it is now very smooth. There
should be a placard to avoid continuous operation at low power
settings within a certain range - can't remember the numbers, since
it's not a range I normally use for any length of time.

Be careful of power off approaches with prop in high pitch. Those 3
blades form a very effective speed brake in idle mode. It's useful to
do a crowbar approach while staying above jet vortices but requires
some practice.