It's probably like Solar said, because you have turned on a DX Preview material. These are really quite bad, and for some reason have to refresh every single texture used in them every time you undo.
I'd recommend using an actual DirectX Shader material type instead (change it from Standard, browse to a .fx file) ... the one that comes with max 2008 (standardfx.fx) is pretty good, but there are a bunch of 3rd party ones on the internet.
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I'd recommend using an actual DirectX Shader material type instead (change it from Standard, browse to a .fx file) ... the one that comes with max 2008 (standardfx.fx) is pretty good, but there are a bunch of 3rd party ones on the internet.