I have a Razer BoomerSpeed (Razer Boomslang 2100dpi), and I have been having issues with vertical cursor movement. I will move the mouse downwards, and, sometimes, the cursor will cease further downward movement. Cleaning the mouse did not fix the problem, and I've taken apart the casing to see what is causing the problem. I've tested the mouse without the top case, and have deduced that this happens when the ball is losing contact with the axle, as the opto-wheel is not spinning. I have tried two other trackballs, including the one it came with (I had replaced the original ball with one from another mouse because of a different issue), and none of them have resolved this issue. Horizontal movement has been unaffected.