Just a couple simple ideas, but publish your game, burn it onto a Cd, and try it in a number of friend's or office machines - see if it is machine specific rather than swopping just video cards in the same machine.
Take your time and observe the results. - notice if they have xp, 98 etc service pack 1 or 2 and so on, if you can a machine it doesn't do it on, that should give you a bit of a hint.
Also, if you or a friend have a spare machine - try starting from scratch and building a virgin machine - one with just the OS, e.g. winXP, service 2, direct X 9c, no other software other than 3dgs.
