Graphics duff on Shuttle SK41G

I have a Shuttle SK41G with on-board graphics - S3 Graphics ProSavage DDR.

I set up the PC using a standard 17" (1024x768) monitor months ago and once
everything was operational and installed nicely, I moved it into my living
room and attached it to the TV (800x600 and/or 640x480) using the TV-out
port, where everything has been fine for a while.

Now, moving it back to a normal monitor and the colours are all wrong!! I
tried changing the display settings, disabled the TV option, upgraded the
drivers and tried everything I could think before resorting to "Format C:"
and re-installing Windows XP. I thought it would be quicker than messing
about with drivers for days, but even with a clean install of XP, installing
the correct drivers causes the screen colours to go crazy, but the
resolution and refresh rates are fine!! If I disable the on-board graphics
in device manager and let the CPU do all the work, the display is perfect
(can't handle any games though), so the cables and screen are fine.

Any ideas???