Happens to me, too. I find that the more out of shape I am, the worse it is.
I don't really belong in this subforum because I'm only 36, but I get it too when I've been off the bike for a while. I think my arms and legs get less used to absorbing the shock from the trail, and I think my back/neck mobility is decreased from not riding as well, my neck sort of "runs out of travel" and can't compensate for the vibrations as easily. I make a conscious effort to keep my shoulders back, my hips rotated forward and my arms and legs loose.
Yes. I have mild astigmatism in my dominant eye. Some sunglasses and safety glasses really mess with my vision. The more curved lenses create a sort of fishbowl effect. Any kind of rough stuff with those makes me feel dizzy. I also wear contacts to correct short sightedness and in certain situations it's enough to vibrate the lenses around.
I've got astigmatism too, I've found that the pupil distance measurement is the main factor that drives the fishbowl type distortion for me. I can get used to it after a day or so, but if I switch back and forth between sunglasses and regular eyeglasses that have different pupil distances, it really messes me up. I ordered my glasses and my sunglasses at the same time from the same place (Roka brand, dope glasses, can't recommend enough) to make sure this didn't bother me and its worked out well. The, um, $600 bill was pretty unpleasant though.
As for contacts, try a different brand. I had the jiggly lenses on my Acuvue Torics, but I've got a set of Bauch and Lomb's that don't move on me and I see annoyingly well. They're not as comfortable as the Acuvues were though.