Problems with a model after texturing

I have several models that I got from somewhere else (I didn't make them). They came as .obj's so I converted them to .3ds in milkshape and then imported them into 3ds max. They look fine when i render them. The problem is that after I texture them (i am using a UVW map), the model will look fine in some views, but often after rendering, it looks as if the faces that you are looking directly at (the ones the would be visible) are missing and that you can see into the model. The faces themselves aren't gone, because if you turn the view around, the ones the used to be "missing" are now visible on the backside. There seems to be another problem as well, because after texturing, the warcraft III viewer crashes when i try to load the model. Am I missing something? Any help would be greatly appreciated.