I get the same thing with reading etc. I kid you not but, what makes it worse for me is bouncy cars with soft suspension. Older Golfs, Vento's were especially bad for me. Cars with a stiffer suspension that don't roll as much, were much better. I also am better when I'm driving, and looking out the window. Bit of air works wonders too. Erratic driving also triggers it. Much better with smoother drivers.
I think its when your body is moving, but you eyes don't see it, when you are reading a book insteading of looking out the window etc. Your brain can reconcile whats happening. Saw a program once showing how they tune rollercosters and simulators to stop people getting the problem. Sinus problems can have similar effects, in my experience. As they mess with your inbuilt sense of balance/motion.
Its a miserable feelng though.
We find kids are less bored when sitting in the middle too.