Ok, I'm texturing a model for my demo reel and having an amazingly hard time with finishing it off. I was all psyched to figure out multiple vertex rgba nodes and have used those with explicit uvw maps for a majority of the model. However, last night I was finishing him up and with what would have been the end of it and did a quick render. For some god forsaken reason now it has huge green and red blocks over the texture. Sometimes he turns a vivid blue instead of his texture color. And on rare occasions black, but the horrible late 60's flashback is the biggest one.

It could be that the normals on your model are corrupted.
Try selecting the entire model and extract all the faces to make a copy.
Make sure to transfer your UVs.
It might help, try rendering the copy...

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Andyba, 3d artist
skills: 3d modeling, texturing, rendering, 3d character animation, compositing
3d software experience: 3ds max, Maya, Softimage XSI, Deep Paint 3d, Body Paint 3D, Wings 3D, Z Brush
2d software experience: Adobe Photoshop, Adobe After effects, Macromedia Flash, Freehand