No doubt you've already done something like this, but on the off chance...

A Google search for "How to avoid motion sickness in FPS game" brought up nearly 20,000 hits. Maybe there's something there that could help?

I'm fortunate in that I don't get motion sickness under any circumstances, but my FPS experience is pretty limited. I am one of the half dozen people in the world who still play DOOM, believe it or not. In terms of playability (not too difficult but still hard enough to be fun) and balance of the weapons (shotgun vs rocket launcher is perfect!) it is the most enjoyable game I have played.

tanstaafl.