You probably invoked xvidtune in root's environment, which likely
doesn't have the proper DISPLAY and XAUTHORITY variables set.
Next time, before you run xvidtune, try setting these:
Then run xvidtune.
Alternatively, instead of getting your root shell with "su -" in
your xterm, use plain old "su" so the xterm retains your DISPLAY
and XAUTHORITY settings.
Most monitors have display controls that might help with minor to