You didn't say the lay of the course, if it is East to West? The problem could be polarization of light through your optics as the sun gets higher in the sky. Any photographer will tell you the sky in the North is always more blue than the Southern sky...if you are North of the Equator. Red and blue wavelengths will change the focusing plane of a lens accordingly and that could effect aim point. Just a possibility. Hope you find the answer.
I have enough problems hitting a 300yd target, no less a 500yd target! Ha..Ha
Nikonut
