Actually, you know what, I don't think it's the Optimus feature now that I think about it.

If it was the Optimus feature handing-off to the Intel GPU mid-game, then not only would I notice a drop in frame rate, I would also notice a change in the graphics themselves, because the Intel GPU would handle certain 3D rendering features differently. Things like dynamic lighting and shadows would change if the game went from Nvidia to Intel mid-game. But that's not happening. The on-screen image in the game looks identical when the problem occurs, the only change is that the frame rate goes down noticeably.

Besides, I don't think games or DirectX could even support the GPU changing out from under them mid-game like that. Could they?

Looking at GPU-Z, I can see that the Nvidia GPU is still the chip that's being used when the problem occurs... it's just that the voltage to the chip has been throttled to increase the battery life of the laptop.