Texture Paint - ctrl+Z destroys all my progress


I’ve started using texture painting in Blender and found it was absolutely perfect for the project I’m working on… except for one thing. Every now and then, when I try to undo a bad stroke, the texture spontaneously turns to solid white. I try to redo, undo again, everything I can think of, but the texture I just spent hours of my time working on is just gone forever. I’ve even tried loading up a previously saved version of the file, but when I do that, the texture image is solid black. I don’t get it. It doesn’t even happen every time; I’ll be having no problems with the undo feature whatsoever for hours at a time, and then I hit ctrl+Z and suddenly, the entire texture is gone and there’s nothing I can do.

I’m running Blender 2.71 under Windows 7 64-bit.

Has anyone else encountered this problem? Also, I’ve been meaning to download Zero Brush for a while. Does anyone know if this issue is still there with Zero Brush?