As TDW pointed out, it's 2 textures being rendered on one spot.
The Grills are a texture with an alpha channel. It's laid right on top of the original Turm texture so you have the 2 conflicting when rendered.
I'd think making a 3D object for the Grills and applying the Grill texture to that would solve the flickering if done right.
A quick way to create the grills would be to do a boolean operation on the Turm useing the texture as a placement guide for the cut.
