It sounds like you have a slow network connection. You can remove that setting by entering the following command in the Terminal:
defaults delete Eggplant AlwaysRequestFBU
I’d first recommend that you try using RealVNC since it tends to work well in its default configuration and doesn’t have a ton of options for how it polls the screen. For UltraVNC, I’d recommend that you turn on “Poll Foreground Window” and “Poll Window Under Cursor”. I’d also recommend disabling the “Video Hook Driver” setting.
EggPlant should generally compensate for slow connections because it waits for a full-screen update before it fails an image search. If it’s requesting one and not getting it from the VNC server (the server is apparently indicating that all the updates have been sent), then the fault lies with the server and not with eggPlant.