Maybe it's worth a shot to try a couple of things mentioned above, as playing at higher resoltion and refresh rates (30Hz? Are you running a CRT monitor?) and increasing your distance to the monitor, so your eyes register the surroundings and and also realize that you are in a stationary situation. Also try not to get yourself too immersed in the game, as this usually amplifies the effects of game sickness. Hope you find a way to soothe the issues.