Sorry about all the questions but,
1. Have you updated to the latest RH8.0 kernel?
2. have you seen any error messages in 'dmesg' regarding the SIS chipset??
I've read that a few people had some problems with the SIS chipset, not sure which ones exactly, but it worth checking..
My board uses the AMD chipset, and it seems to be working ok.. So I can't really help you with the SIS chipset stuff.. Except to find out if there were any errors..