Which aircraft are you flying?
You say that your height varies during the turn, and then when you roll level it goes back to your original height. This could be due to mis-use of the rudder, so you are slipping in the turn. This would cause an error on the static pressure instruments if you only have one static port, say on a cessna 172.
As stated above, don't chase the instruments. Set and hold an attitude, then it should just be a glance in checking the ball is in the middle and the height is holding, then straight back out.