It seems to be a hardware problem than software I guess, but try to download a driver from SIS web site for your VGA and install it again.( be sure to download the driver which match exactly with your VGA model ). Also some monitor have thier own driver too, so install your monitors driver too. I hope this will help.

BTW, also you can change the "refresh rate" of the adapter and see if everything is ok or not. If you don't know how to change it then PM me.

Cheers