I'm actually having the same problem! Well, maybe not IDENTICAL but darn close: I usually have this problem if I go into Stealth mode, or sit down in a chair. The screen goes completely black, except for the menus, and if I stand back up, it goes away. Like you, it hasn't been a big enough problem for me, so I haven't researched it.
jay_l_a, I have similar specs on my rig to yours (same CPU, GPU, same amount of RAM), and I have the same issue. the only way I've seen to resolve it is to set your interior actor shadows to 0. It works for me, but I've heard that Oblivion is sensitive to overclocking, so maybe setting your clocks to default frequencies might help? I may just try that myself. I believe this is probably a videocard problem, perhaps Nvidia will address it in their next driver update. Let's shoot them a message describing our problem.