If it is an Expose/Dashboard issue, it sounds like you could have hot corners turned on. Open System Preferences and choose Dashboard & Expose (should be in the top row). If any of the menus under Active Screen Corners say “All Windows,” “Application Windows,” “Desktop,” or “Dashboard,” that could be your problem (screen saver settings there could be problematic for you, too). I was on a shared workstation with “All Windows” and “Dashboard” set to the two lower screen corners, and it drove me mad every time I tried to drag the timeline.