Some monitors that are not specifically designed for the PC (this is one). Have problems with certain frequencies. 800x600 being the one that most monitors have trouble with.
In the control panel of your video card, see if you can set the frequency of the different resolutions manually. If not, I'm afraid your stuck with making the games work in a different res.
Intel Core2Duo 6400 @ 3.4Ghz
4GB Geil 6400 Ultra
3 x 500 Gb Samsung HD
EVGA 8800 Ultra OC