I have my Viewport Shading set to Textured, and all my textures usually show up fine. However, on some occasions, a texture will stop showing in the Viewport. It will still be assigned to the mesh and everything, it just stops showing. This happens for all instances of the texture at the same time, not just per mesh.
my off hand guess is you running out of video memory. Blender is very resiliant to crashes in my experiance so it keeps going, prehaps at the price a texture or two. though when you render the texture should still be there. It would also make sence why the same texture disapears every were as its been unloaded from the memory.
what your system specs?
Does this happen you get large progects? or an a cube with a small texture?
are you reunning other video accessing items in the background.