I have used Mender 2.0/Meshview/Fragmotion to correct these types of issues for Gameguru and AGK. This is usually bone weight problem. See pic below, where I used Fragmotion to load in the Assassin.X model and selected all, then chose validate weights and assignments under Bone menu. At the 'clean vertex assignments' window. I used the defualt settings and pressed ok. weights will be adjusted. then just export out, maybe rename the original X file. Loaded in AppGameKit and voila, all looks good.