It sounds like you made it sprially unstable by adding too much vertical area to the tail. If it won't track straight and keeps trying to fall off to one side or the other into a spiral dive then that's what you did. Or if it's a close thing and you go into a turn the model will try to tighten into a spiral so that you bank into the turn and then have to hold some constant "top" aileron to prevent the turn tightening. That's a milder form of the problem. Try cutting off the extra you added to the rudder at least. If that fixes the problem then what you need to do if you want more rudder area is to move the hinge line rather than adding area. Pretty much means making a new fin and rudder.