A longshot here but here goes. While I have only been using 3DStudio since 1994 and C++ since about 2001 I am still very fresh to the gaming production end but I am getting there.
I saw this same thing in 3DStudio as well when you are mapping to a cube the two parallel sides show a nice texture but the other 4 are a run of the edge pixels looking like stretching or streaking.
Try making 6 cubes with one dimension so very very small near 0. Sort of like a very flat piece of square cardboard. A piece of cardboard does have a third very small dimension. Position all six of them three dimensionally to build a cube. It would take some fancy for loops to move them in a nice manner all together to maintain the cube shape but it can be done as it worked out in 3DStudio when I created animations using a cube with six different images mapped to the cube.
Just a long shot here,
Luck,
CrypticDragon
You know for a fact that you have reached the edge when you can absolutely see sound!