A couple of thoughts - it might be neck angle - that's what gives me a headache (esp. on my roadbike). First get your position checked - if you are too stretched out, you will have to hold your neck at too sharp an angle. Next, if you are riding with a helmet w/ a peak, take it off (the peak that is) - as it can cause you to crane your neck. Next - if you ride in prescription glasses consider having some sport glasses made esp. for cycling (they grind the focal area towards the top of the lens - also make sure they sit up high enough. Also you might want to consider treating yourself to a really light helmet & don't overdo the tightness.