I should have added that this has occurred over several days, is intermittent and the computer has been restarted more than once in the process of doing other things such as startup from an external FW drive and check permissions and verify the HD, the restart - all of which had nothing to do with the mouse, but in the process of using a laptop in Target mode.
But, that said I'll turn it off.. but the factor that it is intermittent is the puzzle. This is a wired Apple mouse that is 6 years old. Also have just replaced the mouse pad with a different surface to see if that could have been the problem.
If this any Apple Mouse I'd suspect that right off.
One way to test is to Safe Boot from the HD, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, Test for problem in Safe Mode...
PS. Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive
Reboot, test again.
If it only does it in Regular Boot, then it could be some hardware problem like Video card, (Quartz is turned off in Safe Mode), or Airport in 10.4 & below, or 3rd party add-on, Check System Preferences>Accounts>Login Items window to see if it or something relevant is listed.
Check the System Preferences>Other Row, for 3rd party Pref Panes.
Also look in these if they exist, some are invisible...
I remember having a similar issue on my G5 running 10 4 11...
Usually a Sign Out and In would fix it ... for a while... As would a restart...
Apart from BDAqua ‘s excellent advice...
Perhaps it is time for a new mouse...
Just a thought... Have you done a Permissions Repair since
‘ ... using a laptop in Target mode. ‘
Thanks to all.
An answer I typed disappeared. The correct answer/ liked stuff I don't understand so want to say that everyone's answer was correct and helpful. It was also liked.
After changing the mouse pad and cleaning the bottom of the mouse there have been no more crazy movements.
After that I also restarted, re-repaired disk permissions, re-verified the hard drive, delected some unneeded stuff BDAqua mentioned.
There is a Kensington MouseWorks driver that is inactive when it is not attached; and it has not been for many months; rather it's being used on a separate Mac Mini.
So that's the update. All is quiet on the western front.. as the saying goes.