Check your power supply,
as stupid as it sounds I got kicked otu of GPl many times lately becuase
my -5V was at -5.5, apparently when you are shifting the display to 3DFX
mode the card request a sudden burst of power and sometimes....bang, in
extreme cases (like what happened to me last week) the computer stops, not
freezes, STOPS, power off...
I changed my power supply, everything is fine, I am going to race my best
times to improve my GPL rank and i will not tell you because it's only
personal (tell that to all the guys who sign their messages with...handicap
XXX)
Good luck with your power, check it in your BIOS