I have had this problem as well. you can sort of fix it by changing your 3d Graphics Processor option to use OpenGL (or vice versa--can't remember). It really doesn't FIX anything, but GL sort of works differently.
HOWEVER if your GPU does not support GL, then you are stuck with Direct3D. In direct3D, a face can only have 3 sides. hence, in a rectangle, the one side of the rectangle has two faces-two triangles.
Faces MUST HAVE three sides. that's the only way our computers can make them exist.
I hope that helps. Sorry if it doesn't. I can be kind of incoherent sometimes.
if you need anything else, feel free to e-mail me at
Skanker@skateboard.com.
-Sean