So, finally. I'm back with a clean PC after formating and installing a new Windows. I have not yet installed any program at all (except drivers) and first thing I did was installing Myha 1.2 + Xvid. Then I played the game and everything was working perfectly :) Looks like the issue doesn't come from the game: I don't even have Adventure Maker installed on the current config.
I'll take some time soon to think of what could cause the problem on your computer (and others) but the answer is not to seek in the game. That's a good starting point!