I don't have quite the same problem, but when ever I play doom 3, it'll load the level per usual, i'll get into the game (the scene has loaded) and after a very short random interval (try a few seconds max), the game will bring me back to the main screen, and I can't even press 'esc' to get back to where I was! This happens everytime I play, sometimes if I run erratically, I can delay the 'de-loading'.
Only good thing is that it keeps the level data in ram so I don't have to wait very long before I reload the saved game.
Either way, it's a huge pain and I haven't played the game properly since the problem started happening.
Anyone else have this problem?