Black Shading Error On Props

I’m getting black striping on certain props that looks like this:

After I disable phong shading on the material, it disappears.

If it’s a prop_physics, bumping it will temporarily fix the problem. And this happens on GMod and HL2-based games… and I’m completely stumped as to why.

An example VMT:

	$BumpMap	models\props\cs_office\bookshelf1_normal
	$Phong 1
	$PhongExponent 6
	$PhongBoost 1
	$PhongFresnelRanges "[3 3 3]"
	$Detail detail/detail_wood_doskiv
	$DetailScale 8

Check your fresnel ranges…

Tried all 0s, tried 0 0.5 1, that didn’t seem to help.

Edit: I think I fixed it by using the diffuse alpha as the phong mask. shrugs Unfortunately, it gives a less detailed, more diffused lighting effect.
Edit 2: Several friends tested with my textures and got the same results. And the same textures actually work fine in L4D2, so I guess it’s an older Source engine thing.

Might be your GPU starting to die or DX8 is enabled, can you upload the prop?