Known issue with the sis driver contained in Debian's release of 4.3.
The bug was fixed on June 26, 2003 while Debian uses the driver snapshot from June 21, 2003. Tough luck.
Workaround: Go to my website and download a newer driver. Until this is fixed in Debian, you can use my inofficial apt repository for frequent sis driver updates as well as sisctrl (which is not supported in Debian's version at all yet). URL of website below.
Thomas -- Thomas Winischhofer Vienna/Austria thomas AT winischhofer DOT net *** http://www.winischhofer.net twini AT xfree86 DOT org