I've run into this same problem. I believe for me the magic numbers are 5 and 6. So when I am pressing 5 and 6 on the numpad at the same time, I can't adjust my head position. It's no more than an inconvenience to me because I've mapped "Look Up" to a button on my joystick. So instead of pressing 5, 6 and Page Up for example, I press the button on my joystick, 6 and Page Up. I think the problem is more of a Windows thing not wanting to accept too many simultaneous key inputs or something. I don't know.