This scope is a weaver v24 with the fine 1/8 target dot, AO, and focus. My problem is that @ 100 yds my crosshairs disappear if I look at the target for longer than a few seconds. If I look away for a second and back then it reappears but again disappears in a few seconds. The focus was preset at zero and I somehow turned it before I had it mounted. After mounting I tried to readjust it by looking at the sky and back thru the scope but can't see much differance. The focus adjustment is not marked so I can not tell where the zero of the focus is. Any thoughts?