Sounds like it could be too lean on the bottom and rich on the top. I would leave the bottom alone as you have already richened itand lean the top slightly. How did you set the timing when you converted to RCXL? After trying the needle settings you may want to try advancing the timing just a touch. If none of that helps it may just be a charictaristic of the engine with a deep pitch prop on it. The airflow through the prop keeps it spooled up until you are on the ground and slow. My DA 150s used to do the same thing until I learned to slow the airplane down for landing at a slightly higher idle.