[Murph]

Another thing to consider is trim. If your elevator or rudder trim is too far out, the nose of the plane will try to pull up when in level flight. It will also do so if you are flying slowly, as in an extended turn fight.
If you have engaged combat flaps while maneuvering, close them before you try to pull the trigger, they will also cause this "bobbing" effect.
Almost all of the planes in Il2 require some amount of nose-down trim to fly smoothly.