Are you switching windows when it "suddenly drops to 30 fps"? Because the defaults are to limit to 60 in the foreground and 30 in the background, so that would be normal behavior.

Otherwise, it probably depends on what you're doing, or where you are, in the game and I'd need more information to guess at what the "problem" is. 30 FPS is hardly problematic, though. DVDs play at less than that. According to Wikipedia, "Every Hollywood movie is laid to disc" at 23.976 fps