Try and set the same resolution and color depth in the plugin as you have on your X server. For example, if your X server resolution is 1024x768 and 32 bit color, then set that the same for the fullscreen plugin options. This saves the hassle of X resetting the monitor to go to a lower resolution and possibly screwing stuff up.
btw, what plugin and video card are you using?
btw, what plugin and video card are you using?