This may be the same as the problem I'm having, which would mean it's definitely a Blitz3D bug. Nothing to do with compression or image formats.
If two surfaces that use an alpha map happen to be one-in-front-of-the-other from the camera's viewpoint, the renderer gets confused about which is in front and frequently flip-flops back and forth.
Is it possible, nadia, that your model's disappearing polygons are in seperate surfaces? ...I'm trying to think of some way that you can test this to be certain. It's possible that your mesh appears as a contiguous surface, but is actually divided in it's mesh data. If that's the case, then that would match the situation I've described.
Also, what kind of graphics card are you using?
If two surfaces that use an alpha map happen to be one-in-front-of-the-other from the camera's viewpoint, the renderer gets confused about which is in front and frequently flip-flops back and forth.
Is it possible, nadia, that your model's disappearing polygons are in seperate surfaces? ...I'm trying to think of some way that you can test this to be certain. It's possible that your mesh appears as a contiguous surface, but is actually divided in it's mesh data. If that's the case, then that would match the situation I've described.
Also, what kind of graphics card are you using?