It could be the cable as they do go bad. I would try it with a different one. Could be a bad adapter though too. It would be nice if you could try a different monitor like one with DVI. I would also try running your monitor at 60hz and see if that changes anything. What games are you playing that you get 300+ FPS?
More than likely it is due to the fact that the standard for VGA was only designed to use up to 72hz (72 fps) and once you go over 100fps quite honestly it is too fast for the dvi > vga adapter to translate to an analog signal.
Also when playing anything above ~60hz (60 fps) the differences are visually imperceptible. so limiting the output to 100fps is not detrimental to the gaming experience at all.