Jumping cursor

A couple of weeks ago I noticed my cursor was jumping when I was typing. If I was typing a line the cursor would jump to another spot and continue typing there. When I attempt to click at the spot where I want the cursor I just get a drop down menu and this happens several times until I finally get the cursor again.
I have an iMac. I've looked online and the only similar problem is with laptops and the touchpad. That isn't applicable to me.

I should add it happens in any program requiring typing input, Pages, Facebook etc.

i too am having same jumping problem with cursor on my old powerbook g4