Since you set it to 640 x 480, I would say the screen flicker is caused by the refresh rate. Sorry, I'm not a Mamewah user, but I'm sure there is a setting in there to change it. If it is a computer monitor you are using, then I would recommend setting it to at least 60 Hz. If not, then match it to your monitor (arcade?) - 15, 25 or 31 Hz...