i have a similar issue but mine is backwards.
1.0 xbox tsop w/ 4980 bios.
the loading screen is centered perfect on my tv but 2/4 the way thru and then when im inside linux the entire screen is too far right. hoc and voc dont shift the screen
there has to be a way to make linux adjust the screen left or right or up or down.
if i change HOC to anything less than 13 my screen is totally garbled and i have to log in as root and change it back or change the config file directly.
27'' toshiba.
since this is a similar issue with different xbox's and the effect is opposite... there has to be something going on.
i havent tried my svideo cable yet,, not sure if that will help.