320x240 support

I'm trying to use this library with a 320x240 Sharp memory display however it doesn't seems to work properly. Even with:

     #define DEFAULT_WIDTH 320
     #define DEFAULT_HEIGHT 240
     ..
     SHARPMEMORYLCD lcd(320, 240);

I get 3 boxes, side by side displaying the same thing on the top half of the screen - like 3 Arduino logos.  What could be the problem?