Clearly there is a hardware problem that the DeathAdder exhibits when you use both the 900dpi and 450dpi settings. You hit negative acceleration sooner and also get malfunction at lower speeds. My recommendation is to leave the mouse at 1800dpi and not touch the lower resolution settings until this problem has been solved, possibly by a firmware update. So instead of using the 900 and 450dpi settings, simply halve or quarter your sensitivity in-game instead.