I got a similiar ut ed problem right now. I've imported a table, the top of it will be glass so it has an alpha channel. UT is optimizing out tris on the bottom half, which flicker from being drawn to not being drawn depending on your viewing angle.
I haven't set up any antiportals yet, so it's not that. My understanding was that UT only works on a per triangle basis with BSP geometry, and should be optimizing on a per-mesh basis for static meshes. I couldn't find anything that helped in the static mesh browser options, or in the properties for this instance of the static mesh. Any ideas would be helpful, and don't mind the placeholder textures :P