Knoppix6.2en has black MysteryCurtain on iG31/gma3100

MysteryCurtain: desktop draws completely, BUT just before we see Compiz animation, a MysteryCurtain "covers" right side (2 / 5ths) of desktop.

We can still run apps, etc.

Behaviours:
- Samsung sm205bw monitor internal self-report displays "current mode" = 1280x1024
- We see only the Left&Mid 3/5ths of the desktop background
- we can mouse only within the Left-Mid3/5th, where Kllaus' lovely photo is visible
- Again: the mouse itself will not go rightward past edge of MysteryCurtain
- Apps, when maximized use only dimensions INSIDE the background-visible area
- If window is mid-sized, we can drag window rightward to appear to be "under" the BlackBlock
NOW we do a Compiz-kbrd Ctrl_Alt_LeftArrow, and we can see, during animated cube-rotate, the obscured portion of that moved window is rendered "under" the MysteryCurtain.
- same running from usbFlash drive
- same running from hd-installed system

So .. the background is being setup and drawn fully onto a full-sized desktop
Just before we see the Compiz splash, we are beset by MysteryCurtain
Behaves as if we simply covered right portion of Monitor with a black opaque curtain

- - - - - - -
mbrd: asusP5KPL-CM, northbridge: iG31, igp:gma3100
Xubuntu (latest) xfce works fine on this mbrd, as does Debian LIVE LXDE
same Knoppix6.2en install-media ran fine on VIA LN10000(cn700)

Same black curtain symptoms on the right side on my Toshiba R20 Tablet PC running 1440x900. I manually set the res to 1024x768 and curtain is gone, but I'm losing a lot of desktop.