You will find if you do a search on here that the problem you are describing is a well known "feature" of the 4* design. You can help your knife edge by increasing the rudder throw and adding some area to it as well. When you increase the area, increase it more at the bottom than at the top. Like 1/2" at the top and 1 1/2" at the bottom. The other suggested way to "fix" the problem is to cut the fuselage actually lower the horizontal stab some. However, that is a bit drastic.