Sagging rpm tells me it's lean. As Dave suggested, Richen up the low end and I would Richen the high end a half turn or more as well and run it up.
A little back tracking - did you break the engine in to seat the ring or does the engine have at least an hours run time at a rich setting? If the engine is brand new and not broken in, it will sag when set for a normal flight mixture setting. They need to be set fairly fat until the ring seats and seals better. New rings don't seal well and as such need more fuel/oil to help the ring seal.