There are a variety of reasons I can think of. One may be that you have misconfigured the refresh rate of the monitor, or that you are overclocking the processor (has happened to me). I don't suppose you know which server you are using for X? I think there is a specific server to use. Also which version of XFree86 are you using, the latest version may be better. I can't really help any further as I have not had much to do with the virge cards...yet!