Tried all the settings in System Preferences and combinations of them to no avail. Single clicks are interpreted as double clicks. Sometimes just touching it causes an action that should require a click. Positioning the cursor almost always selects an entire word (sometimes an entire line) requiring a forward or back arrow to get to where I want to go. Zapped PRAM. Can't find anything obvious in Library. I'd like to think it's the trackpad, itself, but it could be whatever processes the signals to/from the trackpad. Everything else works perfectly (or at least the same as it always has). I added no software immediately before problem started and don't even remember upgrading anything. Still running 10.4.11 as I have one program that will not run on 10.5+.