I have no idea what's causing that However, the Hammer-view looks funky

Hmmm....is it definitely the textures that disappear/reappear, and not the walls themselves?

check yer distance settings in hammer....# 1 to increase view distance, # 2 to decrease it i believe....been awhile...

Doc Brass...

Given this new information, Doc Brasso is likely to be correct. Here's what he's trying to tell you:

In Hammer, there is a map setting (under "map properties") called maximum viewable distance. By default this is set to 4096. When you load the compiled map in Half-Life, everything more than 4096 units away from you will not be drawn.

You might have accidently set this to a much lower number (say, 128). That could cause your problem. Check this value, and put it back to at least the default of 4096 if it is lower.

There's actually no advantage to having a low max. viewable distance, since although you won't see beyond this distance the engine will still "draw" beyond it (it just won't be displayed). That means that lowering the max. viewable distance can never improve your framerates/r_speeds.

I've only found one use for the max view distance setting, and that's to stop yourself from being able to see out the sky into other parts of your map, for example if you screwed up and your map wraps around but you didn't want it to...

EDIT: Shoot a rocket in there or something and see if the walls go away. Also, map->check for problems, or try moving the light so it shines on the texture instead of above the box.

i'm not sure of exactly how the re-mip-deluxe thing in wally works. but i do believe it has something to do with how a texture is displayed at various distances. when you edit a texture, even thought it looks good, sometimes the small versions that wally makes ( the ones that you might see from 10m+ in game ??) can be corrupted in some way. if the dodgy texture was then exported without any re-mipping, it might result in a problem like the one you are getting now. then again, it might not.

You could be right, I think if you don't click the "re-mip deluxe" in Wally then it won't create smaller textures so you get all black.

I'd go with mipmapping error, though they usually don't work out like that.

Once you figure out how to control the mipmaps in your textures, the wonders will never cease.