I am also having this problem.  I am using Office XP and about a month ago, Excel start locking into scroll mode.  I have been unable to figure out why this is occurring.

It is a frequent but intermittent problem and only related to the mouse.  If I use the keyboard, the problem doesn't occur.  And it's not every mouse movement but most mouse movements.  Once locked into scroll mode, I need to quit the application.  If I "SAVE" the application, the behavior is persistent when I open the file next time.  I then need to delete it and start from scratch

My only solution has been to use the keyboard and save frequently (Control + S).  I have uninstalled and reinstalled the application twice without any luck.