Helping people with computers... one answer at a time.
Bizarre actions from your mouse may mean its imminent death, however there are a few less extreme causes of sensitivity and other odd actions.
My mouse is super sensitive and seems to do things on its own. What do I do?
Most of us have come to rely very heavily on our mouse - even those of us such as myself who are still mostly keyboard users. And when it starts to act up, it can be a very serious issue and an annoying one.
Let's look at a few things.
The most immediate and obvious thing that comes to mind is the hardware itself:
Clean the mouse. Dirt can easily cause the mouse to misbehave. And optical mice are not immune; I find myself needing to clean mine every few weeks.
Check for interference. This applies to wireless mice. It's possible that radio interference could cause some confusion. The thing to look for is a short, clear path between the mouse and its receiver and no powerful electronic devices between.
Replace the batteries. The number one cause for this behavior in wireless mice is dying batteries. Replace them and see what happens.
Check the cable. For cabled mice, a quick test is to wiggle the cable where it joins the mouse and back at the connector, and if that causes activity then the cable is probably bad. Also check for any crimps in the wire that may have also damaged it.
Update your BIOS. If the erratic behavior starts after installing or upgrading to Windows 2000 or Windows XP, then check this article from Microsoft Support: Mouse Jumps or Behaves Erratically After Installing Windows 2000 or Windows XP. The problem might be your computer.
If you have a chance, swap your mouse with another. I have a small graveyard of old mice that I hang onto for just this purpose. They all work, they're just old and dirty, but they come in very handy to determine if a problem I'm seeing is my current mouse or something else.
It is worth checking your settings as well. Unfortunately exactly how will vary dramatically depending on the manufacturer. In Control Panel should be an item labeled Mouse or similar. Items to look for are things like "sensitivity" or "pointer speed". Review the button assignments to make sure that they're correct.
Mouse move options like "snap to" will cause the mouse to move to specific points under certain conditions. These options can easily confuse if you're not prepared for them, so make sure they're what you want or turn them off.
But to be totally honest, the type of wild behavior described is more often than not your mouse's death knell.