This is potentially constrained by the graphics chip, but you know the graphics chip can put out the right timing, as it works under Windows. Of course, maybe you mean that there's no XFree86 driver for that graphics chip and you're forced to use VESA mode and none of those modes fit right, but I doubt that's the case.

Well, I don't have one myself, but the problem is widely reported to be that the chipset operates in two modes: using the built-in VESA modes, none of which is right (the LCD is "widescreen" 1280x768), or "other modes". Programming it for "other modes" is apparently non-obvious and undocumented, and only the closed-source Windows driver knows how to do it.

Peter