Runaway cursor

I am running Word 2000 and have a problem when I am working with several
documents open, switching among them. On a document, I will use the arrow to
go up and down and cursor will start zooming all the way thru article. Can't
stop it. Then cursor disappears or freezes. If I switch to another document
and then back, cursor will reappear.
also have had a problem with Delete sometimes chewing up copy, going wild
deleting everything after it starts. Fortunately I can get it to stop and
then recover by hitting Edit and Undo.
anyone can help with these problems. I've talked to Sony, my manufacturer,
but they don't know anything about it. And the cursor problem only happens in
Word documents, not when doing emails or other things.