Dead Space wasn't 60fps, and was 3rd person.
The OP still hasn't told us what set he is using, and whether it's the same one he uses to play other FPS that don't make him sick.
Dead Space made me fell motion sick. The narrow FOV which made it look like you had zoomed in the image, solid 60fps and motionblur was to much for me.
Marathon 2 for XBLA was another one notorious for churning stomachs. It was the FOV and head bob (not Head BOB) that were thought to be the cause.
Do you have problems with COD2/3/4 as well? I know COD4 has quite twitchy aiming which may be a problem, but if you have no problems with Halo 3, it shouldn't make such a big difference that you are considering stopping playing the game.
However you might try lowering your look sensitivity that might help. What kind of TV are you using btw? Is it the same you use to play H3 on?
Perhaps you could try playing Descent for a while and then a simple FPS will seem like a walk in the park
I never get motion sickness from FPS games, but the FEAR 2 demo with its excessive headbob nearly changed that.
I have played CoD2/3/4, all of which I'm fine with...with prolong session I do feel it coming on. But with WaW, just a short session does it.
I'm playing on a 24" LCD monitor and a Sony 50" LCD TV, both cause the same problem. sigh...
Now that's just an evil game. I remember once it was so bad, I wanted to hurl.
I have a problem with FEAR 2, and the throbbing lights was a painful experience...
Maybe it's a combination of narrow FOV, bobbing head and corridor like environments? As for unstable frame rate causing motion sick; for me I don't think it's a major problem...I don't know maybe it needs to be a combination of a couple of issues to create an issue for me.