The topic that you point to, the main thing that came to me from that was that it appears to have something to do with Joystick/Yoke causing the problem. If you have a Joystick/Yoke, disable it (the default keyboard shortcut is CTRL+K), then use the Pitch Trim Wheels on the A/P to see if that stops it from happening.

Is it possible that it could have something to do with the Sensitivity/Null Zone settings that you have in FSX?

Mark