1st thing I would look at is a set of Wolf springs if you do not already have them.

2nd Mine SBHBH seems to like the 300gr+ cast bullets as well as the 240 XTP at a moderate pace (240 XTP W/ 18.5gr of 2400)

3rd Check the Cylinder throat size

4th The grip pressure comment is very good (Work on holding the gun the same each time)

The best mine has done is about 1.5" @ 50yds

My 45 Colt Bisley will do that out to 100 on a good day that gun is amazing.