Mailarchives (A89: Grayscale troubles)

I'm trying to write my own routine that will fade smoothly between two black and white screens. To do this, I flip rapidly between the screens using the port at address 600010. However, even when I write code that does nothing but flip back and forth constantly, the gray flickers a lot. Does grayscale code have to be synchronized with the refreshes of the LCD to work right? (Then I'd have to use an interrupt... right?) Also, how could I flip the screens to fade smoothly?