Call me old fashioned but I’d suggest speaking to a GP or possibly have some test done at an opticians, it sounds rather odd but I do know that a succession of fast berms that turn one way, then another, and so on can give you a milder version of that feeling you can get on roller coasters that move in the same way, not that I’m saying that’s a bad thing. 8)
The one thing that causes me to feel sick from motion sickness would be prolonged play on a ‘first person shoot em’ up’, especially some of the one’s of years gone by (like Red Faction for example) which have a fairly fast frame rate but dark graphics, not much lighting, combine that with long spells moving quickly through tight corridors and you got a recipe for a sick bag right there.
Although you say you look up ahead on the trail, maybe your line of sight is closer than you think it is because it seems that is one of the causes?