Ditherlacing!! Hopefully the really slow flickering is just my computer display. If not, can someone please help
EDIT: Thank you so much, Mrmo Tarius and Manupix! The picture now flickers properly.
It's an effect called ditherlacing; what you're seeing is two frames where the dithering pattern alternates, so when it flickers at a proper rate it looks like a smooth, third color - in this case, gray.
The effect is neat, but just out of curiosity, why do you want it to flicker?
Old info, but still relevant.
For best results, set frame time to 0.02s :)
I think there's low FPS. Image is really slow flickering. Try to fix it.