This Alpha Behaviour reminds me of some problems I had some time ago. In the first place I blamed Blitz in the Bug Reports, but the problem disappeared when I updated my radeon 9200se's Catalyst driver.
I had terrain troubles (any meshes would lose their brush while the terrain was out of the FOV of the camera), as well as the following alpha problem: all Alpha-Objects disappeared as long as there have been ONLY semi-translucent objects in the scene, so I had to add a fully opaque mesh somewhere to make sure the alpha stuff was rendered at all. And there where a few more strange alpha artifacts.
I had terrain troubles (any meshes would lose their brush while the terrain was out of the FOV of the camera), as well as the following alpha problem: all Alpha-Objects disappeared as long as there have been ONLY semi-translucent objects in the scene, so I had to add a fully opaque mesh somewhere to make sure the alpha stuff was rendered at all. And there where a few more strange alpha artifacts.