Tell us a bit more about your riding setup. Your comment that you're not ready to go to a more upright position indicates that you do know that position can impact on matters as those you've described. However, somthing as simple as a slightly shorter stem or less drop in the handlebars can help take away the strain of holding your head up enough to see while on long rides. Additionally, if it is an issue related to the strain of having to hold your head up while in a more aero position, stregthening the shoulder and neck muscles through resistance training may help. Of course, I would always opt for the opinion of a sports physician if one is available to you. Good luck.
__________________
A conclusion is the place where you got tired of thinking. - S. Wright
Favorite rides in the stable: Indy Fab CJ Ti - Colnago MXL - S-Works Roubaix - Habanero Team Issue - Jamis Eclipse carbon/831