I have used my Red Dot by Millett for the past two deer seasons and have a major problem. The dot is nice and bright when the gun is warm but when I'm hunting in the cold (40 degrees or less) the dot slowly diminishes to almost nothing, even on the brightest setting. I have used about three brand new batteries without any improvements. I am using energizer lithium as instructed. Is the Red Dot junk or is there a better battery?