I've been dealing with the same thing. I recently learned that I tend to come across as arrogant, competitive, aggressive, and overly assertive when I talk, especially when I'm talking about something that interests me. My "stiff posture" only adds to the impression, but I think it's primarily caused by the tone and volume of my voice.
Traditionally, I've controlled this sort of thing by just not talking much, if at all. Now I'm working on being aware of it and trying to consciously control the way my voice sounds. It's kind of like acting.