This issue occurs randomly, but frequently on Conquest on a round by round basis. I've experienced this with all plane types (except the newest bomber as I have not tried it yet).
I have roll left/right key binded to "A" and "D."
I have pitch up/down key binded to "Space" and "Ctrl." Pitch up/down is also Mouse binded to vertical motion (up/down).
In random rounds, the plane pitches and rolls EXTREMELY slowly using the keyboard, to the point where it virtually can't be steered. If I use the mouse to pitch the plane up and down while this issue is occurring, it helps it pitch properly. But the actual keyboard bindings for rolling and pitching won't work (correctly) even if the mouse pitch up/down is working.