S3 ViRGE 325
64 MB RAM
Thanks for any suggestions. This problem never occured before I installed Win 98 (clean install, previously I was using Win95 OSR2 and all was fine). This leads me to believe its a bad driver.
I've seen a similar problem before, and as stupid as it may seem, the problem was that the user had his speakers too close to the monitor...
Since for you it seems to be only when there's a high demand for video quality, I suppose that your speakers may be just out of reach for normal demand, but creat a field that causes the interference that you see when high video quality is needed.