I'm having issues with getting the replay to work, but I have a decent idea as to what issue you're having. My own mouse will suddenly jump on occasion, usually to one of the top corners (often causing me to close something. D:). Although it doesn't have a particular tendency to happen specifically in osu! I myself don't know exactly why it happens, but my best guess is that it's an issue between the mouse and the computer. Try cleaning out/off the mouse to make sure there's nothing physically disrupting the movement of the mouse (I have all sorts of issues when my mouse has something in it, I generally associate this step with improvement all around
). Maybe trying a different mouse as well if you have another might be a good idea, too.