This is a nice glitch feature, actually - and makes it random. Could be used on purpose somehow.

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 :)
Good piece!

I think there's low FPS. Image is really slow flickering. Try to fix it.